The Encyclopaedia contains a brief account of the earlier observations and experiments relative to the production of dew. But the subject was left in a very imperfect state, and would now require, indeed, to be entirely re-modelled. The progress of physical science during the last few years has dispelled the difficulties which obscured that interesting part of meteorology, and has at length furnished a most luminous and complete explication of the whole train of dependent phenomena. We had therefore designed to give some extension to the present article, in the view of entering into the accessory details, and of stating the results of new experiments, performed with scrupulous accuracy, and by the help of instruments of a refined construction. But the late season, (1818,) however grateful in other respects, having proved unfavourable to such researches, we must defer our remarks and calculations till we come to treat of Hygrometry and Meteorology. We are hence obliged to confine ourselves to a sketch of the various opinions that have prevailed concerning the formation of dew, and a short narrative of the successive discoveries made on the subject, till they finally ripened into a consistent and satisfactory conclusion.

Dew is the humidity which the air, under certain circumstances, deposits, in the form of minute globules, on the surfaces of the bodies in contact with it. The Greek term ὑδρος; was evidently derived from ὕδωρ, aqua, implying simply watering or humidification. The Latin name ros is of the same descent. Our English word is obviously borrowed from the German than, akin to the verb which signifies to melt, and conveying the idea, therefore, in the Shaksperian phraseology, of air "melting, thawing, and resolving itself into a dew." The Swedish term dag is no doubt of the same origin, though it likewise denotes low mist or floating vapour. It is remarkable that the French language, though certainly not remarkable for its copiousness, has two distinct terms for dew,—serain, for the humidity which collects in the evening,—and rosée, for what appears accumulated in the morning; the latter being derived from the Latin word ros, and the former intimating that clearness and serenity of the sky which is most conducive to the formation of dew.

When the atmosphere has a temperature below the point of congelation, the dew which might adhere to the substances exposed to it, passes into the form of hoar-frost. This was called by the Greeks πάγος, from its hard or consolidated nature. The French term is exactly the same compound as our own, white frost—gelée blanche. But the German language has a simple and primitive word to denote it, reif; which, in the Swedish, has been slightly modified into rim, a word likewise adopted by the older English writers, and still retained in the Scottish dialect, or dilated into rime-frost, and thence probably corrupted into raw-frost.

As dew appears to collect only during fine clear nights, when the heavens glow with sparkling constellations, the ancients, in the infancy of science, imagined it to be actually shed from the stars, and, therefore, to partake of a pure and celestial essence. Hence the vulgar notion that dew falls, which has prevailed through all ages, and continues to tincture every language. The mythologists described Dew as the daughter of Jove and of the Moon; and Plutarch asserts it to be most abundant in the time of full moon. The lunar beans themselves were supposed to contribute some influence, being of a cold nature, and, therefore, possessed of a humifying quality. The moon, it was imagined, performed merely the office of an imperfect mirror, reflecting the softened lustre of the sun without any portion of his heat.

The dew of heaven has always been regarded as a fluid of the purest and most translucent nature. Hence it was celebrated for that ablergent property which, according to the vulgar persuasion, enables it to remove all spots and stains, and to impart to the skin the bloom and freshness of virgin beauty. Like the elixir of later times, it was conceived to possess the power of extending the duration of human life; and Ammianus Marcellinus ascribes the longevity and robust health of mountaineers, in comparison with the inhabitants of the plains, chiefly to the frequent aspersion of dew on their gelid bodies. Dew was also employed as a most powerful agent, in all their operations, by the alchemists; some of whom pretended that it possessed such a subtle and penetrating efficacy, as to be capable of dissolving gold itself. Following out the same idea, the people of remote antiquity fancied that the external application of dew had some virtue in correcting any disposition to corpulence. The ladies of those days, anxious to preserve their fine forms, procured this celestial wash, by exposing clothes or fleeces of wool to the humidification of the night. It was likewise imagined, that grasshoppers feed wholly on dew, and owe their lean features perhaps to such spare diet.

The philosophers of Greece, after genuine knowledge had illumined that interesting region, entertained far juster notions concerning the nature and formation of dew. Aristotle, whose universal genius ranged over both the physical and the intellectual world, studied facts closely, and sought to reason accurately from the phenomena actually observed. In his book De Mundo, he defines "dew to be humidity detached in minute particles from the clear chill..." In his treatise of Meteorology he states, that "dew is only formed beneath a calm and cloudless sky, but never in windy weather." He farther subjoins, that it collects in low places, and not on the summits of mountains. Vapour, which, according to him, is only heat combined with water, rises in the atmosphere during the day, but when the cold begins to prevail at night, it again discharges its humidity. This vapour, however, he thinks, can never ascend high above the surface of the earth, both because it must soon lose its buoyant heat, and because, in lofty situations, it would be scattered and dissolved by the agitation of the air. Dew is hence most copious in fine weather, and in low damp situations. A north wind checks its production; but a gentle southern gale, charged with humidity, will occasion a copious deposit. When a more intense cold prevails in the atmosphere, the vapour precipitates its humidity in a congealed form, and the dew passes immediately into hoar-frost. Cold occasions this consolidation. Dew has hence the same relation to hoar-frost that rain bears to snow; the frozen mass of clouds constituting the one, and attenuated low vapour, seized by frost, forming the other. The heat of the sun's rays thus first raises the vapour from below; but, in all its subsequent changes and modifications, the moon and stars, contrary to the earlier and more popular notions, exert no sort of influence.

The Aristotelian opinions seem to have given place among the Romans, to the ruder notions which prevailed in remote antiquity respecting its mode of formation. Pliny invariably speaks of dew as falling from the heavens,—cum ros cecidisset. We might expect, therefore, that the poets would continue in their verses to perpetuate the same idea.

Sparsaque celestis rare malebit humum.—OVID, Fast. I. 312. Fitroque maestaria rare, Tempora noctis eunt.—Id. Fast. III. 820. Quae praetexta jacent, Que rarifera mollece aura Zephyrus cornua evocat herbas.—SENeca Trag. Hinc ubi rariferis terram non obreuit umbra. LUCRETI. VI. 4. 64.

Virgil marks the cold which always accompanies the formation of dew, and which, when it becomes more intense, converts the lucid globules into spicular shoots of hoar-frost.

Cum primum gelidos vores aurora resultat.—Eclog. VIII. 15. Pumica roribus minis et pruinis florem amittit.—GEORG.

The opinion that dew falls from the sky, maintained its credit during the course of the middle ages. The alchemists even carried this idea so far as to fancy that, since the dew gradually vanishes in the progress of the day under the action of the solar rays, it then merely seeks, by sympathy, to regain its native seat in the highest heavens. Nay, some of those ingenious enthusiasts have not scrupled, in confirmation of their wild hypothesis, broadly to assert, that a few drops of morning dew, being inclosed in an empty egg-shell, which is placed at the foot of a ladder resting against the roof of a house, the shell will become buoyant while the sun shines, and will mount along the ladder till it reaches the very top. The famous Van Helmont, who refined on the notions of the alchemists, considered the lights of heaven as of two distinct kinds,—the one which flows from the sun and rules the day, being intrinsically warm and possessing masculine virtue,—the other, which rules the night, and emanates from the moon and stars, being of a feminine nature and having a cool or refrigerating influence. This cold light, he imagined, produces the purest essence of water, which is stored in the moon, to recruit the waste of the nether world; and he supposed the allegory of Jacob's ladder might represent that perpetual ascent and descent of aqueous matter, by which the revolution of the system is constantly maintained. That the moon's beams are naturally cold, he thought sufficiently established, by the prevailing belief of the common people, who carefully avoid sleeping in the open air, without some screen to protect them from the chilling impressions which are shot down upon the ground.

Baptista Porta asserted that air is actually converted into water from the accession of cold, and thought this transmutation proved by the fact, that, on the approach of severe weather, the windows of an apartment have the inside of the panes of glass covered with moisture. Gaspar Schott, as late nearly as the middle of the seventeenth century, was so much persuaded of the coldness of the moon's rays, that he stoutly appeals to the effects of their concentration in the focus of a reflector. This experiment, however, was made sixty years before and with an opposite result, by the famous Sanctorio, the founder of scientific medicine, and the inventor of the thermometer. He actually employed the air-thermometer for the first time; but this very ingenious inquirer must have been deceived by some extraneous circumstances, when he saw the liquor sink so considerably as he asserts, under the calorific action of the moon's light. The philosophical ideas of Sanctorio were perhaps in some instances too refined for his age, and the vulgar notion concerning the production of dew continued afterwards, for more than a hundred years, to be still generally retained. But the progress of horticulture near the latter part of the seventeenth century, brought out some unexpected facts, which seemed at variance with the popular belief. It was remarked, that a bell-glass being placed in the evening over a plant, was in the morning profusely covered with dew on the inside, though scarcely any moisture appeared to adhere to the external surface. The humidity which formed the minute

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† Γαλάξι ἐν ἀγρῷ αἰχμαλώτης τε και ἀναπαυεῖ, ἢ γαλάξι ἀγρῷ αἰχμαλώτης τε και ἀναπαυεῖ. globules, must therefore have risen from the plant or the ground, and adhered against the glass.

Such, however, was the very slow advance of sound philosophy, that Perlicius, who proposed what he calls a Drososcope, consisting of an oblique balance playing in a soft rack, for indicating the quantity of dew accumulated in the absence of the observer, concludes the discourse which, under the direction of Professor Weidler, he delivered in scholastic form on taking out his degree at Wurtemberg, in 1727, with the general inference, that Dew descends to us from the atmosphere of Jupiter, Venus, the Moon, Mars, and Saturn; but that, though it falls from the air, it by no means originates in this fluid. He had found in the month of August, that 250 grains of dew formed on the surface of a square foot in the country, while only 94, and at other times 76, or even 64 grains were deposited in the town.

The first person, however, that appears to have openly rejected the inveterate opinion of the descent of dew was Gersten, another German professor, who made several experiments on the subject, and printed at Giessen, in 1733, an Academical Thesis, in which he advanced the opposite hypothesis. He found that all plants exhale, in various proportions, the moisture which forms the aqueous deposit; and remarked, that plates of copper exposed during the night have only their under surface bedewed. This dissertation led the celebrated Professor Musschenbroek to repeat the same observations at Utrecht. Having obtained similar results, he communicated the main facts to his Parisian correspondent, M. Du Fay, who planned immediately a series of experiments on a large scale. This ingenious philosopher caused two tall ladders to be set up, reclining against each other, in a vacant space, remote from all trees and lofty buildings; and on the 25th of October 1736, at four o'clock in the evening, the weather being clear and calm, he laid panes of glass on the steps at the different heights of 6, 13, 17, 25, and 31 feet above the ground. These he visited at certain intervals during the progress of the night. By five o'clock, a pane close to the ground had its under side completely wet, while its upper side was only slightly dewed. At six o'clock, the pane, six feet above the surface, was covered with dew; and at seven o'clock, the effect had reached the highest pane. During the whole night the dew continued to form; but it appeared always more copious on the lower panes.

These facts might be deemed sufficiently conclusive; but M. Du Fay sought likewise to ascertain the relative quantities of moisture deposited at different altitudes. He procured several rectangular bits of green cloth, cut six inches long and four inches broad, and adjusted all to the same weight. These he suspended in horizontal positions at four o'clock in the evening, one of them only half a foot above the ground, and the rest at the heights of 6, 13, and 25 feet. On weighing them next morning, he found that they had respectively imbibed 53, 66, 56, and 54 grains of dew. The subsequent night having proved windy, they gained only 7, 9, 10, and 6 grains. It was evident, therefore, that dew is formed not only sooner, but more copiously, near the surface of the ground than at greater elevations.

To determine, still more precisely, the several quantities of moisture imbibed at different heights, M. Du Fay took three linen towels, each 3½ feet long and 2½ feet broad, and having dried them thoroughly in the sun, he stretched them horizontally at 1 foot, and 17, and 28 feet above the ground. After exposure during the whole night, the air being quite clear and calm, the lowest one was found to have gained 3060 grains, the next only 2346, and the highest 2742. There occurs some slight anomaly in this result; but on reducing the greatest effect to English measures, it corresponds almost exactly to a cubic inch of dew for each square foot of the surface. This might appear to be rather a low estimate for the climate of Paris in the autumnal season, since, at the same rate, it would give only a deposit of about 2½ inches during the whole year.

In the meantime, Musschenbroek made observations on the humification of substances placed above the leaden platform of his observatory at Utrecht. The dew formed in such a situation, he thought, could not have risen from the ground, but must have fallen from the atmosphere. But, pushing these experiments farther, he was conducted to a most curious and interesting discovery. He found that dew forms in very different proportions on different bodies, and that it will scarcely adhere to a surface of polished metal, while it streams profusely over glass or porcelain. Even the colour of the substance appeared, in some instances, to alter the effect. Thus, a piece of red morocco leather acquired, by exposure through the night, twice as much dew as another piece of the same size, whether black or blue. A closer examination, however, convinced him that this modification was not caused by the mere colour itself, but was occasioned by the addition or infusion of the matter employed to produce it.

M. Du Fay, on his part, repeated those experiments with the same success. He likewise performed others of a similar description. Electricity was, at this period, in high vogue, and he had distinguished himself in that career. It, therefore, struck M. Du Fay that the disposition of certain bodies to attract or to repel dew, was somehow connected with the distinction into electrics and conductors. In the prosecution of this idea, he sought to compare the humifying action of vitreous with that of resinous substances. He took a tin basin of the same shape and dimensions as one of glass, and having coated it on both sides with forty or fifty applications of a solution of shell-lac in alcohol, he exposed the two vessels during a fine clear night; but the surface of the glass was found to gather twice as much dew as the resinous coating. In the course of his researches, he likewise noticed another curious fact, the explication of which must be referred to a recent discovery. Having selected two large and equal watch-glasses, he set their convex faces horizontally, the one on a porcelain and the other on a silver saucer, and exposed them both, in this position, to the influence of the night-air; when the former was always observed to collect five or six times more dew than the latter. The metallic surface must have, therefore, in some way or other, contributed by its presence to check the precipitation of humidity, though its direct action is confined to a very narrow limit, since, under a resinous coating of only the hundredth part of an inch in thickness, the effect is precisely the same as with a cake of wax. In confirmation of this remark, having composed a square plate, by cementing at the edge a polished rectangle of brass, six inches long and three inches broad, to a similar piece of glass, he found, on exposing it to the atmosphere at night, the vitreous surface covered as usual with dew; while the metallic one was scarcely at all affected; but, on laying another slip of glass across these plates, the end which rested on the brass remained quite dry, while the other end soon became profusely wetted.

It seemed to follow from the experiments of Musschenbroek and Du Fay, that, strictly speaking, dew neither falls nor rises, but, according to the doctrine of Aristotle, only separates, under a certain change of circumstances, from the air, and attaches itself to some substances in preference to others. The theory of vapour, proposed afterwards by Le Roy of Montpellier, threw farther light on the subject. Moisture is suspended in the atmosphere by a real chemical solution, in the same manner as nitre and other salts are dissolved in water. The solvent energy is in both cases augmented by the addition of heat. A rise of temperature enables the air to support a larger portion of humidity, while the decrease of it enfeebles the attractive power, and occasions a precipitation in the shape of mist or dew. This perspicuous explication, as we have seen, had been already anticipated, though but vaguely stated, by Aristotle.

A deposition entirely similar to dew or hoar-frost is hence formed, whenever the air becomes suddenly chilled, by touching any surface much colder than itself, and not consisting of polished metal. Thus the walls of long passages, vaults, or massive buildings, generally drip with wet during the early part of the summer, before the external heat has sufficiently penetrated. In like manner, it is observed, that when a severe and long-continued frost is succeeded by a thaw, that the backs of houses are quickly incrusted with shoots of hoary icicles.

The formation of dew is hence easily produced artificially in a close room, without waiting for exposure under a clear nocturnal sky. If a caraffe filled with water from a spring or well, be carried into a warm apartment, the outside of the glass will become soon covered over with an aqueous deposit, which must increase till the body of water has acquired nearly the temperature of the encircling air, and will afterwards gradually disappear. But if a piece of tin-foil be applied to the bottle, it will remain dry, while the rest of the surface appears humidified with dew.

It is a curious fact, that air always begins to deposite its moisture on glass, even before it has reached the point of saturation, or become absolutely damp. This property, and the circumstances connected with it, Mr Leslie discovered, in the year 1798, by help of his hygrometer, which he had already brought nearly to a state of perfection. On exposing wine-glasses at the approach of evening, they were soon covered with dew, while this instrument still indicated several degrees of dryness. The difference was yet greater in summer than in winter. The hygrometer being inclosed within a small glass receiver, placed on a wetted plate likewise of glass, stood, after the whole internal surface had become lined with dew, at 5°, in a very cold room, but at 15°, when the apartment was kept warm.

The general observation is explained by Mr Leslie's researches into the propagation of heat. No adjoining substances can ever come into absolute contact; but air approaches much nearer to the boundary of glass, porcelain, or paper, than to a surface of polished metal. By an extension, therefore, of the principle of capillary action, the suspended aqueous particles, which have a strong adhesion to glass, to which they are brought so close, readily detach themselves from their union with the air. But the same particles being held back from the proximity of a metallic surface to which they have little attraction, are never deposited on it unless the air is actually overloaded with them. The hygrometer will, accordingly, reach the absolute zero, when it is shut up within a case of polished tin.

Mr Leslie afterwards made several occasional observations relative to the production of dew, particularly during a series of clear warm days in the month of May 1801, at Easbury in Dorsetshire, where he placed his instruments both on the lawn and on the balustrade of a tower sixty feet in height. Without entering into details, it may be sufficient here to mention the principal results. The periodical variations, both of the hygrometer and thermometer, were much greater near the surface than at some elevation. On the approach of sunset, the thermometer on the ground sunk rapidly; the hygrometer relapsed to about five degrees, and the dew began to form on the blades of grass. During the night, the thermometer descended still lower, the hygrometer indicated absolute humidity, and the lawn was covered with a profusion of dew. But a little after sunrise, the thermometer again mounted, the hygrometer began to act, and the sheet of moisture gradually exhaled. In the progress of the day, the heat and dryness increased; and, about two o'clock, the thermometer and hygrometer, both of them screened from the direct action of sun, stood generally, the former at 75°, and the latter at 85°. But, on the top of the tower, all those changes were less violent. The thermometer, which at that altitude seldom rose in the course of the day to 70°, or the hygrometer to 65°, indicated, as night again closed, a depression, though very moderate in comparison with what was shown at the surface of the ground. The thermometer stood several degrees higher than below, the hygrometer remained at a dryness of 15° or 20°, and no dew was deposited on the balustrade. But similar differences of effect, although on a smaller scale, were exhibited at very moderate heights. On lifting those instruments in the evening only a foot from the ground, the thermometer would rise a degree or two, and the hygrometer mount from the verge of moisture to perhaps 10°. At an elevation of four feet, those changes were nearly doubled. The dew thus always began, as in Du Fay's experiments, to form at the surface of the earth, and continued to mount upwards with the progress of the night.

It is hence easy to explain the general phenomenon of the Phenomena of dew. "In fine calm weather, after the rays..." of the declining sun have ceased to warm the surface of the ground, the descent of the higher mass of air gradually chills the undermost stratum, and disposes it to dampness, till their continued mixture produces a fog, or low cloud. Such fogs are, towards the evening, often observed gathering in narrow vales, or along the course of sluggish rivers, and generally hovering within a few inches of the surface. But in all situations, these watery deposits, either to a greater or a less degree, occur in the same disposition of the atmosphere. The minute suspended globules, attaching themselves to the projecting points of the herbage, form dew in mild weather, or shoot into hoar-frost when cold predominates. They collect most readily on glass, but seem to be repelled by a bright surface of metal.*

The unequal heating of the surface during the day thus occasions, on statical principles, a perpetual interchange between the higher and the lower atmosphere, which is prolonged through the night, the warm portions of air still continuing to ascend, and leaving their place to be occupied by the descent of similar cold portions of that fluid. This vertical play is a provision of nature for the attempting the diurnal vicissitudes of climate. "In clear and calm weather, the air is always drier near the surface during the day than at a certain height above the ground, but it becomes damper on the approach of evening, while, at some elevation, it retains a moderate degree of dryness through the whole of the night. If the sky be clouded, less alteration is betrayed in the state of the air, both during the progress of the day and at different distances from the ground; and if wind prevail, the lower strata of the atmosphere, thus agitated and intermingled, will be reduced to a still nearer equality of condition."†

The descent of chill air caused by superior density, explains the formation of dew in low situations, and its progressive elevation as the cold accumulates. But some farther explication was wanted to reconcile the concluding observations of M. Du Fay. The subject was in consequence resumed by M. Benedict Prevost, who performed a curious set of experiments, described in a Memoir read before the Philosophical Society of Montauban in 1803. The results are certainly perplexing, and would almost seem anomalous. 1. Tin or copper foil, and gold or silver leaf, being applied to plates of glass, and exposed to dew, were observed, as before, to remain generally dry, while the vitreous surface became bathed with moisture. 2. After exposure to the night-air, not only a dry border appeared, extending a little way beyond the film of metal, but the side opposite to that coating continued still dry, though all the rest of the glass was profusely wetted. 3. A piece of glass, being laid above the metallic leaf, destroyed its effect. 4. A rectangular piece of tinfoil being pasted on the inside, at the top of a pane of glass, in a window having a northern exposure, and a similar piece applied at the bottom on the outside; when the dewing began first against the inside, the interior coating appeared wetter than the naked surface, and the portion of this immediately behind the exterior coating seemed always drier than the rest.

The facts were exactly reversed, when the dewing commenced on the outside of the window. 5. Opposite to the middle of a rectangular leaf of metal, a similar but smaller piece being applied on the outside of the pane, when the dew began to form within the apartment, the space behind the exterior coating still remained dry. 6. In all cases, whether on the inside or the outside of the pane, on covering the metallic leaf with a piece of glass of the same dimensions, the effect was exactly the same as if no metal had been interposed. 7. Similar appearances were produced by combining gilt paper or quicksilvered glass, the results depending wholly on the nature of the extreme surfaces, according as they consisted of metal, or of glass or paper.

On reviewing these curious facts, M. Prevost was struck with their apparent analogy to the phenomena of electricity. He thought they might all be comprised under a single proposition: That glass which separates two masses of air of unequal temperatures attracts or repels humidity according as it is armed with metal on the hot or on the cold side. To account for these very singular yet interesting facts, he proposed a random and strained hypothesis, grounded on some loose notions of chemical affinities. But we need to stop to examine it.

Dr Thomas Young, in his Lectures on Natural Theoretical Philosophy, published in 1806, concludes a short abstract of the experiments of Prevost, with suggesting, that they would derive their explication from Mr Leslie's Discoveries on Heat. The anticipation was perfectly just, though the discoveries themselves required then a little farther extension to embrace the whole phenomena. Mr Leslie had carefully investigated the laws which modify the propagation of hot or cold pulses through an aerial medium from a solid or a liquid boundary. But he did not contemplate the pulsation excited at the conterminous surface of two strata of air having different temperatures. It was indeed impossible to devise an experiment in which the opposite layers of fluid could be kept distinct, for the warmer portions of air would seek always to rise, while its colder and denser portions would endeavour to sink downwards, and thus form, by insensible shades, a vertical gradation of temperature. But though the pulsatory action excited at each successive horizontal stratum, might singly escape observation, it seemed probable that the accumulated impressions transmitted from numerous boundaries would become very sensible. Accordingly, in a close heated room, the pyroscope, or differential thermometer having one of its balls gilt, which is susceptible of such pulses only, marks, near the floor, perhaps four or five degrees of calorific impression, yet, when lifted higher, it indicates an effect always diminishing in proportion to the proximity of the ceiling. The entire action exerted, or the amount of the intermediate energies, was therefore, as the excess of the temperature of the stratum of air next the ceiling above that of the stratum in which the instrument happened to be placed. Carried out of doors in clear and calm weather, after the sun had withdrawn his beams, it betrayed a much stronger tendency the contrary way, and marked a copious frigorific impression, evi-

* Leslie on the Relations of Air to Heat and Moisture, p. 132. † Ibid. p. 92. dently produced by the coldness which must pervade the upper regions of the atmosphere. But to fit the pyroscope for making observations during the day, it was converted into the *Aethroscope*, in which the influence of light is neutralized,—a combination of great delicacy, and, therefore, a valuable acquisition to meteorological science. The application of this new instrument has not only ascertained the existence, but measured the intensity, of the cold pulses which are at all times darted downwards from the successive strata of air, though often partially intercepted by clouds, or more completely obstructed by low fogs. But since the spheroidal cup, which concentrates the various oblique impressions on the upper ball of the aethroscope, can do little more than double the direct action against a horizontal surface, it may hence be computed, that, in fine bright evenings, those cold pulses rained from the sky are sufficient alone to depress the temperature of the ground, according to the seasons, sometimes eight degrees, but generally about three degrees, by Fahrenheit's scale. The blades of grass, thus chilled from exposure, cool in their turn the damp air which touches them, and cause it to drop its moisture. For the same reason, the naked ball of the aethroscope, as it is still more cooled, appears much sooner affected, being commonly covered with profuse liquid globules, long before the dew has begun to form on the surface of the ground.

All the difficulties and seeming anomalies of the observations of Du Fay and Prevost now vanish away. The various phenomena proceed chiefly from the cold, induced by exposure under a clear sky; but other causes will often essentially modify the results. 1. The impression received on a plate of polished metal scarcely amounts to the tenth part of what is communicated to a surface of glass, wood, cloth, paper, earth, or grass. 2. When the action continues the same, the corresponding depression of temperature yet depends on the slowness with which the cold is subsequently dispersed. In calm weather, a plate of glass, or a sheet of paper, if covered on both sides with a leaf of metal, will gain or lose heat twice as slow as before; and if coated only on one side, its progress will be a half slower. But high winds greatly assist the dispersion of heat, and often reduce the effects of external impressions to the third or the fourth part of their ordinary measure.

Hence the reason why scarcely any dew is formed in windy weather, though the sky be clear; for the frigorific pulses must then have little efficacy, not cooling the ground perhaps more than one or two degrees. In the last observation of Du Fay, the slip of glass laid across the rectangle, composed of alternate bars of glass and of brass, being greatly chilled by exposure, had by contact communicated its coldness to the matter under it, and thus enabled the metal to assist in the deposition of dew. In Prevost's second experiment, the metallic leaf being scarcely affected by the frigorific impressions, checked by its presence the progress of cold along the vitreous surface, and therefore maintained a dry border all around it. Hence, in his third experiment, a piece of glass covering the metal received the entire impressions, and restored the former effect. The application of a metallic coating against the inside of a pane, must, in the fourth experiment, have augmented by one half the efficacy of the external pulses of cold, and thus made the dew to attach more profusely. For a like reason, while a leaf of metal on the outside of a pane became, in the fifth experiment, slightly dewed, the addition of a smaller metallic against the inside increased the effect, by promoting the accumulation of cold.

The remarkable experiments which the late Dr Patrick Wilson, Professor of Practical Astronomy in the University of Glasgow, performed during the severe frost of January 1780, are easily explained on the same principles. In the declivity of a garden, a thermometer laid, in a clear starlight, on the surface of the snow, stood from eight to ten degrees lower than when suspended at the height of a few inches. This excessive cold was evidently not occasioned by evaporation, for, on blowing with bellows against the bulb when it lay on the snow, so far from sinking more, the mercury actually rose two degrees higher than its station in the free air. The intensity was, no doubt, in part owing to the low position of the snow, for a thermometer suspended at a pole projecting from a window 24 feet above the surface, indicated four degrees less cold than below. But, besides, the accumulating action of the descent of cold air, the snow must have been also chilled extremely by the frigorific pulses darted from an azure sky. This inference, though not perceived at the time, or, indeed, likely to have been admitted then as philosophical, is distinctly supported by an experiment of Dr Wilson. Having screened a spot of the garden by a sort of sharp roof formed with two inclined sheets of brown paper, and laid a thermometer under it on the surface of the snow, the instrument soon marked 6 degrees of less cold than before, or than another exposed at only a short distance. But this open screen, since it could not impede the mere descent and influx of cold air, must have intercepted a more powerful frigorific influence.

Dr Wilson afterwards performed other similar experiments, which are detailed in his paper on hoarfrost, drawn up in 1788, and inserted in the first volume of the *Transactions of the Royal Society of Edinburgh*. He made the important remark, that during a fog there was no difference of temperature between the surface of snow and the incumbent air. But he neglected to pursue the consequences, and was disposed, from the various facts which he had observed, to conclude vaguely that hoar-frost is always accompanied by a production of cold.

About the same time Mr Six of Canterbury, the inventor of the self-registering thermometer, employed that very useful instrument in making similar but more extensive observations. He found, in a clear summer evening, his thermometer, when laid on the grass, to sink 5 degrees lower than when suspended freely near the surface. But he had occasion afterwards to remark still greater differences.

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*See article Climate, p. 197, &c. of this volume, and also Vol. VIII, p. 465, &c. of the Transactions of the Royal Society of Edinburgh.* On a clear and still night in winter, the thermometer which had been supported in the air, fell no fewer than $13\frac{1}{2}$ degrees when placed flat on a meadow. He likewise noticed, as Dr Wilson had done, that thick fogs always impede, and often wholly prevent, the peculiar cooling of the ground.

It seemed, therefore, ascertained, that, in the absence of the sun, the surface of the earth, and especially its projecting herbage, acquire, in calm weather, from the mere aspect of a bright and unclouded sky, a very notable degree of cold. This cold appears likewise connected evidently with the formation of dew. But what is the nature of that relation? Is the coldness contracted by substances on exposure to the nocturnal air, to be considered as the effect or as the cause of their dewing? The former opinion, we have seen, was espoused by Dr Wilson, though sound theory should make us expect, that the deposition of dew, or the conversion of humidity from a gaseous to the liquid state, must, on the contrary, occasion a small extrication of heat. But constant experience shows, that cold bodies, not sheathed with metallic lustre, become always sprinkled with minute aqueous globules, from the contact of damp air. The simplest truths, however, are very seldom the soonest perceived; and the late ingenious and learned Dr Wells has the merit of being the first who distinctly attributed the formation of dew to the previous cold induced on the ground from the aspect of the sky. He had early conceived an opposite idea, but a closer examination of the subject led him to adopt juster views. Being once engaged in the research, he prosecuted his observations with assiduity and ardour for upwards of two years, at a friend's villa on the skirts of London, in spite of his professional avocations, and at the evident risk of his precarious health. The numerous facts thus collected, are detailed in his Essay on Dew, which appeared in 1814, and immediately attracted a very considerable share of public notice. This little work, however, does not add much to our stock of accurate information; but it is rendered interesting, by the variety of collateral objects which it embraces. The experiments themselves rarely display address or delicacy; and Dr Wells, without ever employing the hygrometer or the pyroscope, instruments which he could have then easily procured, generally contents himself with stating merely rude approximations. Fortunately, such coarse results were sufficient to support the main principle, for otherwise they would have required much correction. But we must still regret that the worthy author should have frequently trusted to conjectural reasoning, instead of appealing to direct experiment.

The chief observations collected by Dr Wells may be reduced to a narrow compass. The coldness of the objects exposed was always found to precede the formation of dew, which continued, in favourable circumstances, to accumulate somewhat progressively during the whole night, so that, from midnight to sunrise, the deposition was even greater than from sunset to midnight. Dew was more abundant in the spring and autumn than at other seasons, and it was always very copious when the atmosphere inclined to humidity—for instance, in clear nights succeeding to misty mornings, or in clear mornings succeeding to misty nights.

The coldness which bodies contract from exposure must be augmented by every circumstance which retards the communication of heat. Hence loose and spongy materials are mostly affected. Thus, in a clear night, the grass was 12 degrees colder than the garden mould, and 16\(\frac{1}{2}\) degrees colder than a hard gravel walk. In another bright evening, the surface of snow being 9 degrees colder than the air, a piece of swandown laid on it became still 4 degrees colder. Again, a lock of wool, placed on a small table in the garden, became 9\(\frac{1}{2}\) degrees colder than the air, while swandown, in the same situation, acquired a coldness of 11\(\frac{1}{2}\) degrees.

The quantities of dew which attach to different substances appear to follow the proportions of their relative coldness. Parcels of wool, each weighing ten grains, being teased out into flattened balls of 2\(\frac{1}{2}\) inches diameter, and laid on a grass plot, on a gravel walk, and on fresh garden mould, acquired, during a clear calm night respectively, 16, 9, and 8 grams of humidity. In another favourable night, ten grains of wool laid on the table attracted 16 grams of dew; while another similar parcel, suspended at the same height in the free air, acquired only 10 grams; but the former must have also been much colder than the latter, since its confined situation, unlike the open exposition, would check the dissipation of the frigorific impressions. Hence dew is always denser on grass than on the leaves of shrubs.

But the cooling of substances from exposure, though one great source of dew, is not the only cause of its formation. In low fogs, while the ground is scarcely colder than the incumbent bed of air, the humidity yet settles profusely on all bodies, even on the polished surface of metals. From Six's experiments it appears that, from the height of 200 feet, the temperature of the atmosphere, in fine evenings, decreases regularly about 10 degrees, the colder, and therefore denser portions, being always thrown down to the surface. Hence the reason of the ancient remark, that dew is more copious in low vales than on the tops of hills. But the observations of Dr Wells serve to confirm the general statement. A lock of wool exposed on a table, imbibed, during a clear night, 16 grams of dew, but a similar parcel, placed immediately under the table, and consequently screened from the aspect of the sky, attracted 4 grams. In the latter case, the mere accumulation of cold air below, must have occasioned the aqueous deposition.

It might perhaps have been judged sufficient, if Dr Wells had contented himself with assuming the coldness induced on the ground as merely an experimental fact. At any rate, we cannot help regretting that he should have sought the explication of this primary phenomenon from the very loose, cumbrous and visionary hypothesis of M. Prevost of Geneva, concerning what is gratuitously called radiant heat. We are at a loss, indeed, to conceive how a speculation, so repugnant to all the principles of sound philosophy, should, at this time, have procured any favour, unless it proceeds from the blind admiration which the multitude are prone to entertain for whatever lulls the reasoning faculty, and appears cloudy and mysterious. In the body of the Work, under the title Materia Medica, is inserted a general view of the subject of Dietetics, and a pretty complete list of alimentary articles. The discoveries made in chemistry since that article was written, as well as the progress of physiology connected with the subject of nutrition, will, however, enable us to give some interesting additions on a subject of great importance to every one, without being obliged to treat it again in a regular system.

The necessity of aliment is explained by a knowledge of the functions of the body, and its selection depends upon the same principles. The living machine, as well as those that are inanimate, wastes in proportion as it is used, and this waste must be supplied. To learn the kind of supply required, the kind of waste and its mode must be ascertained.

The human body is of a very compound nature; indeed, it is the most compound of all bodies, as well as the most complicated of all machines. It is composed of solids and fluids, and these again consist of various chemical elements in different states of combination. A great part of the mass of our bodies consists of water, and certain animal substances, to which chemists have given the name of fibrin, albumen, gelatin, mucus, and osmazome. Our bones consist principally of phosphate of lime. Besides these, some other principles enter into the composition of our bodies, though in comparatively small proportion. All the elementary matters, of which these principles consist, are continually discharged by the various excretions, but generally in states of combination different from those in which they existed as a part of our body. By the lungs a great deal of carbon and hydrogen is exhaled in the form of carbonic acid gas and vapour; by the skin carbon and hydrogen are also thrown off in considerable quantity; by urine, in addition to carbon, hydrogen, and oxygen, much azote, phosphorus, and lime, are discharged in the form of urea, and the phosphate of lime; and by the alvine evacuation, not only the indigestible parts of our aliment are expelled, but also carbon, hydrogen, and azote, which formed integrant parts of our bodies, and have fulfilled their functions, in the form of bile, mucus, and intestinal flatus. We, therefore, see that there is a constant waste of carbon, azote, hydrogen, oxygen, phosphorus, and lime going on, which must be replaced. But there are only two sources from which this waste can be repaired,—the atmosphere in which we live, and the aliment which we introduce into our stomach. The atmosphere consists of oxygen and azotic gases, and it is very doubtful whether any part of either be absorbed or converted into a part of our bodies. At least we may assume that, from the air, no part of the materials to supply the waste of the body is derived. These must, therefore, be furnished entirely from the matters introduced into the stomach, and those which are calculated to restore any of the deficient elements or principles alone are alimentary. It is not at all necessary that these elements should be in the same state of combination with the principles whose loss they are to supply. It is sufficient that the elements be there, for it is the very essence of the function of digestion to analyse the alimentary matters, and reunite their elements into other combinations assimilated to our nature. From this view of the subject it would, however, seem that the more nearly the alimentary substances approach to the nature of the substances whose waste they are to supply, the less change upon them is necessary, and their digestion and assimilation will be more easy. Upon these principles, animal substances should be more easily digested than vegetable, and a larger proportion of their elements should be assimilated, while a smaller proportion should be separated to form excrementitious or indigestible compounds. In the same manner, vegetable substances are more digestible, and generate less excrementitious matter than inorganic substances, which furnish only a small proportion of assimilable matter, and which must be separated from combinations totally foreign to our nature.

Besides alimentary substances properly so called, there is another class of substances which do not contribute much to repair the waste of our bodies, and yet perform an essential part in the function of digestion. These are called condiments, and their use is to stimulate the organs of digestion to greater activity, and, in fact, they are all much more sapid than the proper alimentary substances, which are in themselves generally insipid or mawkish.

From the view we have taken of aliments, it will appear that they are furnished by all the kingdoms of nature; the mineral kingdom supplying chiefly water and lime, while the vegetable, in addition to these in smaller quantity, yields much carbon and hydrogen; and the animal kingdom, in addition to a proportion of all the preceding elements, furnishes almost all the azote which enters into our composition. Although this statement be generally true, there are facts which, at first, do not seem to accord with it; and there are some grounds for believing, that living bodies have either the power of changing the elementary nature of bodies, or of analysing these bodies we at present consider as simple, so that one is apparently changed into another. Thus some animals, in the state of nature, live only upon animal substances, and it is easy to conceive how, by a very simple process, the blood and flesh of their prey should become a part of their proper blood and flesh. Their elements, and even the combinations of them, are alike. But there are other animals whose flesh and blood do not differ materially from those of carnivorous animals, and which live almost entirely upon vegetable substances, far removed from animal nature, and containing little if any azote. This subject has lately engaged the attention of Magendie, the most distinguished Parisian physiologist of the present day; and his views are the most important lately promulgated upon this point, and throw very great light upon the subject of dietetics. To ascertain the sources from which animals derive the azote which enters into their bodies, he performed some experiments which appear to prove, that azote is an indispensable constituent in the food of animals. For the subjects of his experiments he chose dogs, because, like man, they can be supported by vegetable as well as animal food, and he confined them to the use of pure water, and substances totally devoid of azote.

Sugar, perfectly pure, was first tried. Of this, and of distilled water, he allowed an unlimited quantity to a small dog, three years old. For the first seven or eight days it seemed to agree very well with this diet. It was lively, active, and eat and drank as usual. In the second week it began to fall off, although its appetite continued very good, and it eat from six to eight ounces of sugar in the course of twenty-four hours. Its alvine excretions were scarce and scanty, while that by urine was abundant. In the third week it became more emaciated, it lost its liveliness, and its appetite began to fail. During this period also, its eyes became affected in a singular and very distressing manner. The emaciation increased every day, its strength failed, and although it continued to eat from three to four ounces of sugar daily, it became so weak that it could neither chew nor swallow, and of course could not move. It died on the thirty-second day of the experiment; and, on opening its body, there was a total absence of fat; the muscles were reduced to one-sixth of their bulk, and the stomach and intestines were much contracted. The gall and urinary bladder were both filled with fluid; but on analysis, the bile and urine resembled those of herbivorous animals. The urine, instead of being acid, as in those which eat flesh, was like that of herbivorous animals, sensibly alkaline, and did not contain a trace of uric acid or the phosphates, while the bile contained the picromel so remarkable in ox gall. The excrements also contained much less azote than usual. This experiment was twice repeated, with nearly the same result.

Olive-oil was next tried with two healthy young dogs, which seemed to agree with them for the first fifteen days, but then produced the same bad effects, and both died on the thirty-first day.

Gum was given to several dogs, and always with the same result.

Butter, an animal substance, but which does not contain azote, was also tried; and although, after the thirty-second day, the dog was allowed as much meat as it could eat, it died on the thirty-sixth day, similarly affected.

M. Magendie also killed several dogs, at a proper period, after they had got a full meal of oil, sugar, or gum, to observe the nature of the chyle thus furnished. The chyle of the oil was of a decided milky white, while those of the gum and sugar were transparent, opaline, and more watery. These experiments, in M. Magendie's opinion, render it doubtful whether the oils, fats, gum, and especially sugar, are so nutritive as is generally supposed. But, before we adopt his conclusion, we must remember that whole nations subsist upon food which contains very little, if any azote. The Hindoos live almost entirely upon rice; the peasants of Lombardy upon maize; those of Ireland upon potatoes; the slaves in the West Indies get fat during the cane crop, and the negroes of Senegal during the gum harvest, and herbivorous animals are nourished at all times upon grass. M. Magendie is not ignorant of these facts, but tries to explain them away by doubting the accuracy of some of the relations, and alleging that few vegetables are altogether destitute of azote. He cites, in confirmation of his observations, the experiments of Dr Stark, who injured himself by trying to live on sugar, bread and water; and of M. Cloutet, who grew extremely weak upon potatoes and water, and instances the insufficiency of sugar and a little rum to support the crew of a shipwrecked Hamburgh vessel. The legitimate conclusions, from all the facts relating to this subject, seem to be,

1. That animals derive the azote which enters into their composition entirely from their food, and hence, that no animal can live for a considerable time upon food totally destitute of azote.

2. That animals, even those naturally carnivorous, can live a certain time upon food entirely destitute of azote, in consequence of which, the excretions of the naturally carnivorous become altered, and throw off less azote than when fed on animal food, acquiring the properties which these excretions have in animals whose food contains a very small proportion of azote.

3. That vegetable and animal substances, destitute of azote, are highly nutritious, provided, at the same time, azote be supplied from the admixture of some other aliment containing it, though in small proportion.

Upon these principles, alimentary substances may naturally and philosophically be divided into three great classes.

I. Those which contain azote, carbon, hydrogen, and oxygen.

II. Those which contain carbon, hydrogen, and oxygen.

III. Those which contain neither azote nor carbon.

1. Alimentary Principles which contain Azote, Carbon, Hydrogen, and Oxygen.

The aliments which contain azote correspond with the animal substances in general, and are calculated to repair the waste of our solids and fluids, without great alteration or effort in the digesting organs. All the immediate principles of this class are not, however, equally digestible, or possessed of

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* Memoire sur les propriétés nutritives des substances qui ne contiennent pas d'azote, 8vo, Paris, 1816. the same properties. We shall say a few words of each.

1. **Fibrin** constitutes the great mass of the solid matter of the muscles of animals, especially of those which are old and have dark-coloured dry flesh. It is also a principal constituent of the blood of all animals. There can be no doubt, therefore, that it is pre-eminently nutritious in these its natural forms of combination, but we know of no experiments to ascertain its nourishing powers when used alone. The purest form of fibrin which occurs in common circumstances, is the fibre of flesh which has been long boiled in a great quantity of water, as in the remains of the meat from which beef-tea is made, or of that boiled down for soup. This is generally considered, and is often thrown away, as totally indigestible, and deprived of all its nourishing principles: but this is probably a vulgar error, for animal fibre in this state still contains, as much as ever, all the elementary substances which are necessary for animal food; and the only circumstance which can account for their indigestibility, is their great aggregation, which it is the business of cookery to overcome. Fibrin also forms a large proportion of the substance of some of the internal organs of animals, all of which are nutritious. Pure fibrin is white and opaque when moist, but acquires a dark colour on being dried. It does not become putrid when kept in the air, nor even when immersed in water for a considerable length of time. It contracts and shrinks on the application of heat, and gives out, on being burnt, the smell of burning horn or feathers. It is insoluble in cold water—is corrugated by boiling in water—is insoluble in alcohol, but strong acetic acid swells it considerably, and renders it transparent like cartilage, in which state it may be dissolved, or, at least, diffused through water by long boiling.

Fibrin varies in every species of animal, and in the same animal at different ages, either from a difference in its nature, or from a difference in the matter with which it is combined. In many fishes, and the lower classes of animals in general, it is semi-transparent and colourless. In veal, pork, salmon, chicken, and some other kinds of poultry, it has a pink colour; in beef and mutton it is of a fuller red; and in pigeon and game, both birds and quadrupeds, it is dark coloured. In general, it is more tender in the female than in the male, and in the young animal than in the old.

**Albumen** is also a principal constituent of animal substances, in which it exists in two states, one uncoagulated and the other coagulated. Of the former, the purest example occurs in the raw white of egg. Cartilage, horn, hair, nails, consist chiefly of the latter. It is also a principal constituent of blood and brain; and it seems to be the chief substance of oysters, mussels, and snails. Uncoagulated albumen is sometimes solid, often glairy, always transparent, and, when fluid, is soluble in water, and its taste is bland, or almost insipid. At 165° Fahrenheit it is converted into a white solid mass, of which we have a familiar example in the white of a hard boiled egg. There can be no doubt that albumen, especially in its uncoagulated state, is highly nutritious, and easily digestible.

The curd of milk may be considered as a variety of albumen, although it possesses some peculiar properties, especially that of being converted into cheese by a particular mode of management.

**Gelatin** is a third very principal constituent of animal solids, as bones, ligaments, tendons, membranes, skin, muscles, &c., and exists in much larger proportions in the flesh of young than of adult animals. Thus we see the gravy of veal and lamb always gelatinize, while that of beef and mutton does not. The swimming bladder of the several species of sturgeon is gelatin in a state of very great purity, and by boiling it may be extracted pure from the shavings of hartshorn. Its taste is bland, and nearly insipid. It is characterized by its solubility in water being much increased by a boiling temperature, and by the solution, when of a certain strength, gelatinizing or cooling. It is highly nutritious, and supposed to be the most easily digestible of animal matters.

**Mucus** differs from albumen chiefly in not being coagulated by heat or corrosive sublimate, and from gelatin in not being precipitated by vegetable astringents, nor gelatinizing when its solution is concentrated. It exists nearly pure in saliva, and is a constituent of most of the secretions. There can be no doubt of its easy digestion and nutritious quality.

Of these four principal constituents of animal matter we may remark, that in themselves they are almost insipid; that gelatin exists almost entirely in a solid form, more or less dense; that mucus and albumen exist in every form of aggregation, from perfect fluidity to the density of cartilage; and that fibrin is only fluid in the living blood, but in every other instance is a tough solid; and that gelatin is very soluble in boiling water, and gelatinizes on cooling; that albumen is soluble in cold water, and coagulates at 165°; and that fibrin is not soluble either in cold or hot water. We may also remark, that, although chemists have given very definite characters of each, as if they constituted absolutely distinct species of matter, these characters are taken from certain selected kinds of each, and that, in reality, we find that there is a regular and insensible gradation from mucus, through gelatin and albumen to fibrin, and that, as in the process of animalization, as well as in the progress of life, they pass into each other, and many intermediate states are found which cannot be distinctly referred to any of them.

**Osmazome**, or animal extractive, differs very much from the preceding principles; chemically, in being soluble in alcohol, and to the senses, in being very savoury or sapid. It is upon this, which seems to admit of considerable varieties, that the flavour of animal food, and of each of its kinds, depends. It exists chiefly in the fibrous organs, or combined with fibrin in the muscles, while the tendons and other gelatinous organs seem to be destitute of it. The flesh of game and old animals also probably contain it in greater quantity than that of young animals abounding in gelatin.

**Gluten** is the only vegetable substance which contains a notable proportion of azote in its composi- When separated from other principles it forms a tough, ductile, elastic, and tenacious mass of a grey colour, resembling, when drawn out, thin animal membrane; when dried it is brittle, hard, and slightly transparent like glue. When kept moist it ferments and acquires some of the properties of cheese. Immersed in water it at last putrefies. When burnt or distilled it resembles in its properties horn or feathers. It is soluble in concentrated acetic acid, and by the assistance of heat in muriatic acid, and in the alkalies. It then bears a strong analogy to the animal substances in general, resembling, by different properties, fibrin, albumen, gelatin. It is very generally found, though only in a small proportion, in the vegetable kingdom, in all the farinaceous seeds, in the leaves of cabbages, cresses, &c.; in some fruits, flowers, and roots, and in the green feculum of vegetables in general, but it is particularly abundant in wheat, and imparts to wheat-flour the property of fermenting and making bread. On the nutritious powers of gluten separated from other principles, nothing certain is known; but the superior nutritious powers of wheat-flour over that of all other farinaceous substances, sufficiently proves that, in combination with starch, it is highly nutritive, and in all probability it is the gluten of the green feculum which supplies the azote necessary for the support of the herbivorous animals.

2. Alimentary Principles which contain Carbon, Hydrogen, and Oxygen.

Starch is very abundantly diffused through the vegetable kingdom. It exists in great purity in various farinaceous grains, such as rice, barley, maize, and millet; it is combined with gluten in wheat; with saccharine matter in some grains, as oats, and in many leguminous seeds, such as haricot-beans, lentils, vetches, and peas; with viscous mucilage, in rye, potatoes, and Windsor-beans; with fixed oil and mucilage in the emulsive seeds, such as nuts, almonds, cocoa, tamarinds, linseed, rapeseed, hempseed, poppyseed, and, in general, all those from which an oil can be obtained by expression. Lastly, starch is sometimes united to a poisonous substance. Of this singular union of a nutritious with an injurious principle, the most remarkable instance occurs in the roots of the Jatropha manihot, and of many species of arum, to the former of which the negro slaves of the West Indies are indebted for their cassada bread, and from the latter is prepared the best arrow-root starch. Only one species of grain, the Lolium temulentum, is hurtful, but many leguminous seeds are poisonous, of which the most familiar example occurs in the laburnum peas.

Starch is artificially prepared in great purity from various substances. Starch is got from wheat and potatoes; arrow-root from various species of arum; cassada-flour from the manioc root; satep from the orchidese in general; sago from the pith of various species of pal-trees; tapioca from the bitter and sweet cassava root. In all of these varieties of form, starch furnishes a bland and wholesome nutriment.

Gum or Mucilage is also a principal ingredient in the composition of our alimentary vegetables. The distinctive character of gum is its solubility in cold as well as hot water, and its insolubility in alcohol. It is devoid of smell, and to the taste it is bland and agreeable. In Arabia, Senegal, and the East Indies, it is obtained in great quantities from the various species of Mimosa, from the bark of which it exudes in great purity; and in hot climates, in general, it is furnished by many trees, especially those which have an astringent bark. In our own country, an example of its production is seen on the bark of the plum and cherry trees. Where it is produced in sufficient quantity, it constitutes a principal article of diet; and the Africans of Senegal are said to live entirely upon it during the gum harvest. Eight ounces of gum are the daily allowance, and furnish sufficient nourishment for each man.

Mucilage is the alimentary principle of many of our esculent vegetables. In some it is united only to green colouring matter, as in the leaves of beet and spinach; with bitter matter, which may be prevented by the process of blanching, as in endive, lettuce, succory, and cardoon, or by using the plant very young, as in asparagus. It exists also in every part of the mallow tribe; in many roots, as scorzonera, salsify, and Jerusalem artichokes, in the receptacle of the flower of the artichoke. It is combined with an acid in sorrel leaves; with saccharine matter in many fruits, as the fig and date; in roots, as the carrot, parsnip, and beet; and with slight acrimony in the turnip, cabbage leaves, cauliflower and broccoli; and with considerable acrimony in the radish, cress, and mustard. It exists in great quantity, combined with a peculiar nauseous principle, in onions, garlic, shalot, leek, &c.; and, lastly, in small quantity, with much aroma, in those vegetables which are used only for seasoning, as parsley, thyme, &c. In short, it is very generally found throughout the vegetable kingdom, and in every mode of union with other principles.

Sugar, the common properties of which, in a state approaching to purity, are familiar, is also highly nutritious. It is crystallizable, soluble in water both cold and hot, in alcohol and the weak acids, readily undergoing, when dissolved in sufficient water, the vinous and acetous fermentation, but, on the other hand, when concentrated, preserving vegetable substances. Chemically considered, it presents many varieties. It exists in greatest quantity, combined with mucilage, in the juice of the sugar cane, of the maple tree, the manna ash tree, and of beet-root. It seems to be a constant attendant upon the inflorescence of vegetables, for almost every flower furnishes honey to the bee, and is a chief constituent of all the acerb, subacid, and sweet fruits, in combination with vegetable jelly. Sugar is produced, or at least collected, by several insects. To the bee we are indebted for honey, and a species of locust in New Holland covers the trees and ground with a kind of sugar. In all animals a principle having some analogy with sugar exists in the bile, and it is a product of morbid action in the disease called Diabetes.

Oil and fat are also nutritious. They differ most obviously in fluidity, and they coincide in being in- soluble in water, and in containing a larger proportion of hydrogen than the alimentary matters already spoken of. The oils may be divided into the fluid and concrete, and both are furnished by the vegetable and animal kingdoms. Fluid oil exists in quantity in the emulsive seeds, in some of them combined with prussic acid, as in the bitter almond, and in others with an acid matter, as in the seeds of the ricinus, but it is obtained in greatest quantity and purity from the olive. The animal fluid oils are all more or less nauseous, as spermaceti oil, seal oil, whale oil, and cod liver oil. The concrete oils are generally furnished by the animal kingdom, and these are often bland and agreeable when fresh, but are apt to become rancid in proportion as they are less solid. Butter is the least consistent, if we except the fat of some birds; then hog's lard, the subcutaneous fat of beef, and the kidney fat of beef and mutton in succession. The only concrete oil obtained from the vegetable kingdom is the butter of cocoa.

3. Alimentary Principles which do not contain Carbon.

Water is perhaps the only really alimentary substance which belongs to this class, but it is one of the most essential. It is not only necessary to replace the constant waste of water which is drained off from our bodies, by the secretions, the cuticular discharge, and the vapour of the breath, but is in itself strictly digestible, and capable of supplying either hydrogen or oxygen to the system, as may be required, according to the nature of our other food. When we consider how large a proportion of the whole weight of our bodies consists of water only, and that the fluids require more frequent renewal than the solids, the necessity of water as an aliment cannot be disputed. Some animals, as the rabbit, are supposed to be capable of living a long time or altogether without water; but this is a mere deception, for their vegetable food consists almost entirely of water. On the other hand, Dr Fordyce kept gold fishes six months in distilled water, and thought himself warranted in concluding, that animals could live in water and air alone. Pouteau allowed some of his patients nothing but water for several weeks, without their failing off; and the histories of shipwrecked mariners prove, with how small a portion of solid food man can subsist, provided he has sufficient allowance of water, while without water or a substitute, no quantity of solid food can support man for even a few days.

Earths are, perhaps, not altogether unalimentary. Not to mention the depraved appetite of many young females, and of the dirt-eating negroes of the West Indies, for chalk, cinders, and such substances, earth is sought after and devoured by whole nations. The luxurious Capuans paid a considerable tribute to the Neapolitans, for an earth called Leucogaeum, which they considered necessary for the preparation of a favourite dish, Alicia. The Tunguses, according to Laxmann, eat a fine clay with rein-deer's milk. Chandler saw the women and children in Samos chewing pieces of steatite as a luxury. La Billardiere Dietetics saw the same practised in New Caledonia, and found edible earth sold in the market in several villages in Java. Throughout all India lime is used along with the betel leaf. Kepler partook of the butter earth, which is eaten with great relish, spread upon bread, by the millstone quarriers of Thuringia; and, lastly, Humboldt has made us acquainted with the existence of a whole nation of earth eaters, the Ottomacs on the Orinoco.* We might also adduce that bird-fanciers find it necessary to supply birds shut up in cages with sand and earth. All these facts, we are aware, might be explained upon principles different from the digestibility of the earthly substances taken into the stomach, and we have no idea that any earthly substances can supply carbon or azote to the system; but we have absolute proof that earthly matter may enter into the circulation, in the growth and absorption of the bony frame of our body, for which phosphate of lime is as necessary as carbon or azote for our soft solids.

Sea Salt is more obviously necessary than earth. Even in insular and maritime situations it is voluntarily used as a condiment by all, but it is only in inland countries, at a distance from the sea, that its necessity is duly appreciated. Muriate of soda enters into the composition of all our fluids, and is thrown off by many of our secretions; hence its waste must be supplied, and where the vegetables are not naturally impregnated with it, it becomes one of the most indispensable articles of our food.

Alimentary substances, as presented to us by nature or prepared by art, may be considered in various points of view.

They differ in regard to digestibility, or the facility with which they are decomposed by the powers of the stomach, to enter into new combinations fitted to repair the waste of the blood. In this particular, also, they may differ in respect to the length of time, or in regard to the digestive power of the stomach, required for their digestion. Thus the digestion of one substance may be slow, though ultimately complete, even in a weak stomach, and that of another quick enough in a strong stomach, although imperfectly digested by one that is weak. In reference to their digestibility, aliments are commonly described as being light or heavy, but in this respect there is very great difference in regard to different individuals, the same substances being light to one and heavy to another, and vice versa.

Mr Astley Cooper made some experiments to ascertain the comparative digestibility of different kinds of raw meat without fat; and the following table exhibits the loss 100 parts of each sustained in the stomach of dogs, which were killed, one, two, three, and four hours, after being fed.

| | 10 | 20 | 98 | 100 | |-------|----|----|----|-----| | Pork | | | | | | Mutton| | | | | | Beef | | | | | | Veal | | | | |

* Tableaux de la Nature. Par A. Humboldt, 2 tomes 12mo, Paris, 1808. In another experiment, after four hours, the digestibility appeared in the following order: Cheese, mutton, pork, veal, beef. Fat appeared to be also much more digestible than cheese; beef than potatoe, and codfish than beef. Boiled veal was much more digestible than roast; and of different parts of the same kind of food, the digestibility was in the following order: Fat, muscles, skin, cartilage, tendon, and bone.*

From the experiments detailed in the inaugural dissertation of Dr Macdonald, *De Ciborum Concoctione*, Edinburgh, 1818, which were made in company with the late Dr Gordon, there appears to be great irregularity in the time necessary for the completion of digestion, so that they scarcely furnish any conclusion as to the comparative digestibility of different substances. Dr Macdonald infers that, of those he tried, butter was the most, and rice the least, digestible in the stomach of the dog. In the experiments which Dr Stark made upon himself, to ascertain the nutritious properties of oily substances, he found that, with a daily allowance of 30 ounces of bread and 3 lb. of water, two ounces of olive oil taken at one meal was so large a quantity as to be disagreeable; three ounces in the day caused some uneasiness in his bowels; and four ounces gripped him very much, although he gained weight; but this experiment was not conclusive, as at the time he was suffering under sloughing gums, the effects of a protracted diet of sugar. A diet of four ounces of pure fat, obtained from the subcutaneous fat of beef, made into a pudding, with twenty ounces of flour, and twelve or twenty ounces of water, with the remainder of three pounds of water in drink, was both nourishing and agreeable, but when the fat was increased to six ounces, great part of it passed unassimilated, and his bowels were affected. The same pudding without the suet was not sufficiently nutritious, and did not satisfy his appetite in the same manner. When the pudding was made with butter, although only four ounces were used, he was made very ill by it. Oil of butter agreed very well, and oil of marrow, of all the fats Dr Stark tried, he found to be the mildest in the bowels. His gums having again become purple and swelled, with pectechial appearances on his body, while making these experiments, suggested to him the following queries, which seem important to the science of dietetics. "Although at present I take more food than what is absolutely necessary for the support of the body, I remain perfectly well, whereas I have several times suffered considerable inconvenience from committing any excess in the quantity of oils. Is it not evident, that excess in the quantity of oils is more hurtful to the body than excess in any other article of food? and that, of course, we ought to be particularly careful in regulating the quantity and quality of the oils we employ in diet?"

Is it not probable then, that animal oils, though they nourish and increase the weight of the body, are not of themselves sufficient to prevent a morbid alteration from taking place in the blood and fluids? whilst, on the other hand, the lean of meat, though less nutritious, is of more efficacy in preserving the fluids of the body in a sound state?"†

Aliments also differ in regard to the proportional difference quantity of nourishment they furnish, and, in this point of view, they are said to be strong and weak, or rich and poor. This difference may arise either from the proportional quantity of digestible and indigestible parts in the various kinds of aliment, or from the digestible parts being different in kind, and furnishing a supply of a different kind to the blood. There is even in this respect some opposition between light food and strong food, and it may be generally observed, that food which is most quickly digested, requires the soonest to be repeated, while digestible food, that is only slowly digested, supports the body for a greater length of time.

Aliments also differ in the impression they make on our palate; and it is chiefly in this respect that they are considered by the epicure. This impression proceeds from two distinct qualities in the aliment; the one depending upon their grosser physical properties, and the other upon their finer, recognizable only by the senses of taste and smell. To the former class belong the sensations of solid and fluid, hard and soft, tough and tender, crisp and stringy, hot and cold, greasy, glutinous, gritty, smooth, &c. These are judged of by the tongue and palate, rather as organs of touch than of taste, and are altogether independent of flavour, as capable of affecting the organs of taste and smell. The latter class consists of all the variety of tastes, properly so called—sweet, bitter, sour, salt, alkaline, astringent, aromatic, nauseous, pungent, acid, spirituous, cooling, &c. and also the want of taste, the vapid or mawkish. Of these, some are almost universally agreeable, and others generally disliked, but much depends upon idiosyncracy, state of health, education, habit of the individual, and upon the degree or quantity of flavour.

Aliments also differ in the impression made upon the stomach; but the sensations arising from this source are more obscure and less varied. Except the sensation of heat, which may arise from caloric, and is transient, or from acrimony or spirit, which is more durable, most of the sensations experienced in the stomach are indications of its mechanical state, or of affections of the appetite. Hence we have the feeling of gratification, from removal of a sense of emptiness, of repletion, distension, cessation of hunger or thirst, satiety, and sickness.

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* A Treatise on the Nature and Cure of Gout and Rheumatism, including General Considerations on Morbid States of the Digestive Organs; some Remarks on Regimen, and Practical Observations on Gravel. By Charles Scudamore, M. D. 8vo. London, 1817.

† The Works of the late William Stark, M. D. consisting of Clinical and Anatomical Observations, with Experiments Dietetical and Statistical, revised and published from his original Manuscripts. By James Carmichael Smyth, M. D. 4to. London, 1788. We should also consider the effect of different kinds of diet, when the body is in a state of health, and different states of disease; but accurate experiments are still wanting, to enable us to give anything more than fragments of this interesting subject. It is extremely difficult to institute these experiments satisfactorily. They are irksome to the person on whom they are tried; and so many causes tend to interfere with the results, that it is only by frequent repetition that the real effects can be fairly deduced.

Our diet may be either proper, or it may err, and this either in quantity or quality. When the quantity is too small, the body is not nourished, it becomes lean, the fat disappears, and the muscles either get soft and flabby, or shrivelled and dried up, accompanied by loss of strength or stiffness, with predisposition to an actual disease. Errors in regard to the quantity of food are merely relative; so much depends upon circumstances, as individuality of constitution, period of life, state of health, degree of mental and corporeal exertion, habit and temperature. Each person may be said to have a different standard quantity, deviations from which are to be accounted errors. In our army, the rations allowed for each soldier at home are, 3 lbs. of meat, boiled so as to afford broth, with 1½d. worth of potatoes and other vegetables, 1 lb. of bread, or 1½ lb. of oatmeal, and in most cases 1 lb. of milk or coffee is purchased for his breakfast. On service the rations are 1 lb. of meat, 1½ lb. of bread, and 1 pint of wine, or ½ pint of spirits.

Mr Buxton states, that the diet allowed to the prisoners in the jails in London varies from fourteen ounces of bread per day, and two pounds of meat per week, which, he says, is not enough to support life, up to one pound and a half of bread, one pound of potatoes, two pints of hot gruel, and either six ounces of boiled meat, without bone, and after boiling, or a quart of strong broth, mixed with vegetables, per day, which is as much more than enough; and Mr Buxton thinks that the meat should be discontinued. A fit prison diet, in his opinion, should consist of one pound and a half of bread, at least one day old, to each prisoner daily, and one pint of good gruel for breakfast; and, upon good behaviour, half a pound of meat on Sundays.*

Some experiments have been made to ascertain the quantity of different kinds of food necessary for the sustenance of individuals. Dr Franklin, when a journeyman printer, lived a fortnight on bread and water, at the rate of 10 lb. of bread a-week. Dr Stark, whose weight was 171 lb. avoirdupois, found that 38 ounces of bread daily were not more than sufficient to satisfy his appetite; 48 ounces were the utmost he could consume in one day; and the greatest quantity he could take at one meal, without uneasiness, was 30 ounces; and, with this diet, he required necessarily 3 lb. of water for drink; for with only 2 lb. he was not satisfied. In another experiment, 30 ounces of bread, and 3 lb. of water, with 6 ounces of boiled beef, sufficed; with 4 ounces of the beef his appetite was not satisfied; with 2 lb. of bread, and 3 lb. of infusion of tea, he found that 1 lb. of cold stewed beef was not more than sufficient; he was not satisfied with 4 ounces of beef to breakfast; but 8 ounces at dinner, and 4 ounces at supper, were rather too much.

Absolute starvation produces diminished excretions, fetid breath, foul skin, and death. The most distressing histories of this dreadful end are recorded in the account of shipwrecks, and of those unfortunate persons who fall into the hands of the Arabs of the desert. Man can sustain the absolute want of food for several days, more or fewer in number according to circumstances; the old better than the young, and the fat, probably, better than the lean. The total want of drink can be borne only a very short time, and its effects are more distressing than those of want of food. They have been strikingly described by Mungo Park and Ali Bey, as experienced in their own persons. The narratives of shipwrecked mariners also prove with how very little food life may be supported for a considerable length of time; and the history of those impostors who pretend to live altogether without food or drink, display this adaptation of the wants of the body to its means of supply in a still more striking manner; for, even after the deception, in such cases as that of Ann Moore, is exposed, it will be found that the quantity of aliment actually taken was incredibly small.†

Captain Woodard has added to his interesting narrative many instances of the power of the human body to resist the effects of severe abstinence.‡ He himself and his five companions rowed their boat for seven days without any sustenance but a bottle of brandy, and then wandered about the shores of Celebes six more, without any other food than a little water and a few berries. Robert Scotney lived seventy-five days alone in a boat with three pounds and a half of meat, three pounds of flour, two hogsheads of water, some whale oil, and a small quantity of salt. He also used an amazing quantity of tobacco. Six soldiers deserted from St Helena in a boat, on the 10th of June 1799, with twenty-five pounds of bread and about thirteen gallons of water. On the 18th, they reduced their allowance to one ounce of bread and two mouthfuls of water, on which they subsisted till the 26th, when their store was expended. Captain Inglefield, with eleven others, after five days of scanty diet, were obliged to restrict it to a biscuit divided into twelve morsels for breakfast, and the same for dinner, with an ounce or two of water daily. In ten days, a very stout man died, unable to

* An Inquiry whether Crime or Misery are produced or prevented by our present System of Prison Discipline. By Thomas Fowell Buxton, M.P. 12mo. Edinb. 1818.

† An Examination of the Imposture of Ann Moore, called the Fasting Woman of Tutbury, illustrated by Remarks on other Cases of Real or Pretended Abstinence. By Alexander Henderson, M.D. 8vo. London, 1813. Also Rev. Legh Richmond, in Medical and Physical Journal, by Samuel Fothergill, M.D. and William Royston, 469, Vol. XXIX. 8vo. London, 1813.

‡ The Narrative of Captain David Woodard and Four Seamen. 2d edition, 8vo. London, 1805. swallow, and delirious. Lieutenant Bligh and his crew lived forty-two days upon five days' provisions.

In the tenth volume of Hufeland's Journal, M. Gerlach, a Surgeon-Major of the Prussian Army, has related a very remarkable and well authenticated case of voluntary starvation. A recruit, to avoid serving, had cut off the fore-finger of his right-hand. When in hospital for the cure of the wound, dreading the punishment which awaited him, he resolved to starve himself; and on the 2d of August began obstinately to refuse all food or drink, and persisted in this resolution to the 24th August. During these 22 days he had absolutely taken neither food, drink, nor medicine, and had no evacuation from his bowels. He had now become very much emaciated, his belly somewhat distended, he had violent pain in his loins, his thirst was excessive, and his febrile heat burning. His behaviour had also become timid. Having been promised his discharge unpunished, he was now prevailed upon to take some sustenance, but could not, at first, bear even weak soup and lukewarm drinks. Under proper treatment, he continued to mend for eight days, and his strength was returning, when, on the 1st of September, he again refused food and got a wild look. He took a little barley-water every four or five days to the 8th; from that day to the 11th he took a little biscuit with wine; but again from the 11th September to the 9th October, a period of 28 days, he neither took food, drink, nor had any natural evacuation. From the 9th to the 11th he again took a little nourishment, and began to recruit; but, on the 11th, he finally renewed his resolution to starve himself, and persevered until his death, which took place on the 21st November, after a total abstinence of 42 days.

On the other hand, the quantity of nourishment that can be devoured with impunity is often very great. Almost every person in good circumstances eats more than is necessary for supporting his body in a state of health; and many bring their stomachs to require a very excessive allowance as almost necessary. In some individuals an inordinate appetite seems constitutional. Charles Domery, aged 21, six feet three inches high, and well made but thin, when a prisoner of war at Liverpool, consumed in one day 4lbs. of cow's udder, and 10lbs. of beef, both raw, together with 2lbs. of tallow candles, and five bottles of porter; and although allowed the daily rations of ten men, he was not satisfied.*

Baron Percy has recorded a still more extraordinary instance, in a soldier of the name of Tarare, who, at the age of 17, of moderate size, rather thin, and weighing only 170lbs., could devour, in the course of twenty-four hours, a leg of beef 24lbs. in weight, and thought nothing of swallowing the dinner prepared for fifteen German boors.† But these men were remarkable, not only for the quantity they consumed, but also for its quality, giving a preference to raw meat, and even living flesh and blood. Domery in one year eat 174 cats, dead and alive; and Tarare was strongly suspected of having devoured an infant, which disappeared mysteriously. Many other histories of the same kind are preserved; and although some of the individuals were men of large stature and great strength, others were of ordinary size. The excess of food may be taken either in the form of too much at one meal, or of too many meals. It is either digested and furnishes an excess of nourishment, or it passes through the canal simply indigested, or it undergoes the fermentation natural to it. An excess of nourishment either produces a great or rapid increase of the size of body generally, or of the fat and abdominal viscera in particular, or by inducing great fulness of blood, produces diseases which sometimes counteract the effects of the plethora. When the excess passes simply indigested, it only occasionally proves hurtful as a mechanical irritation in the bowels, especially when it is of a hard substance, and has sharp angles. When it undergoes its natural fermentation this is either acid or putrid, as the substance is vegetable or animal, or rather as it is destitute of, or contains a notable proportion of azote.

When diet errs in quality, it gives rise to a greater variety of cases. It may either produce a directly hurtful effect upon the constitution, in the manner of a poison or medicine, in its natural state, or after fermenting in the stomach; or it may prove injurious more indirectly by not supplying an element necessary for its healthy condition, or by supplying one in excessive proportion. The poisonous effects of alimentary substances are always occasional, and arise from a peculiarity in the aliment itself, as in the case of poisonous fishes, or in the individual, as in those persons who cannot eat particular kinds of food, which are, to others, wholesome and nutritious. The unpleasant effects of substances undergoing their natural fermentation in the stomach, are much more frequently observed. They occur either from a very strong disposition in the food to ferment, so that the action of a healthy stomach is not able to restrain it, or from excess of the food, so that part of it is left to its natural changes, or from weakness of the stomach, which exerts little action upon it. Fermenting substances are hurtful, by acting as direct poisons, and by distending the stomach; in the non-azotized substances becoming acid and producing flatulencies, in the azotized substances becoming putrid and producing fetid eructations and flatus. Diet, which errs by supplying one of the elementary constituents of our body in excess, or in not supplying another, does not produce its full effects at once, but gradually changes the condition of the body. When an elementary principle is furnished in excess, it is thrown off by the various excretions; and hence we find, that the urine of omnivorous animals, when confined to animal food, con-

* Account of a man who lives upon large quantities of raw flesh, by Dr Johnston; in Medical and Physical Journal by Drs Bradley, Beatty, and Nohden, Vol. III. 8vo. London, 1800. † Memoire sur la Polyphagie. See Journal de Médecine, Chirurgie, et Pharmacie, par MM. Corvisart, Leroux, et Boyer, Tome IX. 8vo. Paris, An. xiii.