are those which bear a considerable degree of heat without evaporating, or losing any of their weight. Among the most fixed bodies are diamonds, gold, &c. See Diamond, Gold, &c.

**Fixed**, or **Fixable Air**, an invisible and permanently elastic fluid, superior in gravity to the common atmospheric air and most other aerial fluids, exceedingly destructive to animal life; produced in great quantities, naturally from combustible bodies, and artificially by many chemical processes. From its apparently acid properties it has obtained the name of *aerial acid*, *cretaceous acid*, and *carbonic acid*; from its noxious qualities, it has been called *mephitic air*, or *mephitic gas*; and, from the circumstance of being produced in vast quantities during the combustion of charcoal, it first obtained from Van Helmont the name of *gas fulvus*. The term fixed air has been given from its property of readily losing its elasticity, and fixing itself in many bodies, particularly those of the calcareous kind; and though some objected to the propriety of the term, the fluid in question is so well known by the name of fixed air, that we choose still to retain it.

The nature and properties of fixed air are explained under the article **AEREOLOGY**. It is there considered as an acid, and the reasons for supposing it to be composed of phlogiston and dephlogisticated air are set forth. In a paper of the Manchester Transactions by Mr Delaval on the permanent colours of opaque bodies, he considers the nature of fixed air, and gives an account somewhat different. He denies its acid property, which has been generally looked upon as so well ascertained. "The change of colour produced in vegetable juices by the electric spark (says he), is adduced as a proof of the acidity of fixed air; but it has been already shown, that this does not arise from acid, but phlogistic matter." The acid quality of fixed air is also generally inferred from its power of dissolving iron. But phlogiston is also a solvent of iron. Thus a considerable portion of that metal is always dissolved and held in solution by the phlogisticated alkaline lixivium, which consists of inflammable matter calcined with fixed alkali. M. Margraaf has shown, that several other metals are soluble in this lixivium. Hence it is evident, that the solubility of iron does not prove the acidity of the solvent, but may arise from the phlogiston contained in it.

"Fixed air is also supposed to be an acid, by diminishing the causticity and promoting the crystallization of fixed alkalies; but this hypothesis does not agree with the effects which are produced by the combination of acids with alkalies. By these combinations neutral salts are produced; but alkalies do not become neutral by combination with fixed air, being only changed by such an union from caustic and deliquescent to mild and crystallizable alkalies: whence it is evident, that the alteration in them produced by fixed air is not to be attributed to the introduction of an acid.

"We must therefore (continues our author) turn our eyes to the consideration of some other principle by which these effects may be produced; and this principle appears to be phlogiston. The phlogisticated alkaline lixivium is perfectly mild when saturated; and by a slight evaporation is reduced to a concrete crystalline mass, which does not deliquesce or imbibe the least moisture from the air, and no longer retains any alkaline character or property. M. Beaumé, by an elegant and ingenious experiment, has proved the presence of phlogiston in mild alkalies; and has shown, that their power of crystallizing depends upon their union with that principle. He heated in a silver vessel a lixivium of mild alkali, which imparted to the silver a covering or coating of inflammable matter, by which its surface was tarnished and became black. The lixivium was several times poured out of the silver vessel; and after the surface of the metal had been freed from the tarnish, the lixivium was replaced in it, and again heated, by which the tarnish was renewed. This was repeated till the lixivium no longer communicated any stain to the silver. The causticity of the lixivium increased in proportion as it imparted its phlogiston to the silver; and at the end of the process the alkali became perfectly caustic and incapable of crystallizing.

"Those instances, and many others which might be adduced, seem to prove that the change which fixed air produces in caustic alkalies is not effected by acid but phlogistic matter. It is certain, that the matter communicated to lime by fixed air is the very same which it imparts to alkalies: for it may be transferred, unchanged, from one of these substances to another; and when united to either of them, still retains the same qualities. Therefore, if phlogiston renders alkalies mild, and effects their crystallization, the same principle also precipitates lime, and in like manner restores it to its state of mild calcareous earth.

"The experiments and observations on which Dr Black has established his comprehensive and consistent theory, clearly prove, that lime is precipitated from lime-water by fixed air; but his views were not extended to an investigation of the particular matter or quality whereby fixed air operates that effect.

"Lime, which has been precipitated from lime-water, and restored to the state of a mild calcareous earth, is again soluble by the addition of a larger proportion of fixed air; which has been considered as an additional proof of the acidity of the latter. It has also been considered as an extraordinary circumstance, that two such opposite effects should be produced by the same substance. Fixed Air. The simplicity observed by nature in her operations, however, will not allow us to suppose that fixed air is composed of two different or opposite qualities, by one of which it precipitates, and by the other it dissolves. The precipitation of lime from lime-water, and its resolution, are effected by an equable uniform action, exercised by one and the same principle, which is a constituent and essential part of fixed air. Such a precipitation and resolution are not extraordinary or complex phenomena, as has been thought, but are analogous to the ordinary and constant effects which arise from chemical affinities. This may be exemplified by any compound which assumes a concrete and solid consistence by its union with a given quantity of fluid, and which by the addition of a larger quantity of the same fluid is reduced to a liquid state. Thus, when a due proportion of water is added to iron and vitriolic acid, a mutual attraction takes place between these three ingredients, by means of which they are united; and, by their combination, a concrete vitriol or metallic salt is formed. But if a greater quantity of water be added to this concrete salt, as the mutual attraction after this addition subsides equably between the vitriolic salt and the whole mass of water, the acid and ferruginous particles are more minutely divided, and diffused uniformly throughout every part of the water. Thus the solid concrete salt is resolved, and a vitriolic liquor is formed, in which the water predominates.

"Lime strongly attracts and unites with inflammable substances, as sulphur, camphor, and resins. Fixed air has a still greater affinity with it; because, in all the more gross substances, the phlogiston is alloyed with salt, earth, and other matters: but in fixed air it exists in a purer, and consequently a more active state.

"As alkalies are rendered mild or caustic by the presence or absence of the inflammable principle, it can hardly be doubted that the difference between mild and calcareous earth and quicklime is also occasioned by a communication or deprivation of the same principle.

"The origin of fixed air seems to prove its phlogistic nature; for all bodies which yield it, yield also inflammable matter, but many of them do not yield any acid. Calcareous spar, magnesia, and alkaline salts, send forth fixed air; and all these substances, by the loss of it, are deprived of their inflammable contents. Diamonds, exposed to the focus of a burning glass under a receiver, impart to the air contained in it a power of precipitating lime from lime-water when it is agitated with it: But it does not appear that any acid can be derived from these bodies.

"Some of the properties of fixed air are consistent with either the character of an acid or phlogiston. Such are, its power of altering the colour of vegetable juices; its affinity to alkalies, and ready union with lime; its power of dissolving iron, which is inflamed in all acids, and likewise in the phlogisticated alkaline lixivium. The antiseptic quality prevails equally in acids and in inflammable spirits. Acids are disengaged from substances which are decomposed by stronger acids; phlogiston is likewise expelled from bodies which dissolved in acids.

"The qualities of acid and phlogiston agree in these and several other instances; but fixed air is endowed with properties which are peculiar to phlogiston: such as its power of effecting the crystallization of alkalies without changing them to neutral salts; its tendency to escape from water; and its affinity with the air, by means of which a considerable quantity of fixed air is united with and diffused throughout every part of the atmosphere.

"Water, as well as phlogiston, is a constituent part of all substances which yield fixed air. Both these principles have a strong affinity to air. This appears from the union which air forms with the inflammable principle when it is disengaged from bodies by combustion, fermentation, putrefaction, or any other mode of decomposition; and from the mutual attraction of water and air, which is manifested by evaporation, and by the constant presence of aqueous particles in the atmosphere.

"The laws of chemical analysis will hardly permit us to doubt that the air which is obtained from mild alkalies, calcareous earth, and various other substances, receives from them, when they are decomposed, the same contents which were united in them as constituent parts while they were in their entire state; and their analysis invariably shows, that air, water, and phlogiston, enter their composition.

"Hence it seems to follow, that fixed air consists of these three ingredients, either united in bodies, and discharged from them already combined, or that it is formed in the atmosphere by the concurrence and union of these principles; and the phenomena both of fixed and phlogisticated air may be solved by the action and properties of these ingredients.

"The weight of fixed air indicates that it contains a considerable portion of aqueous matter; and it is by means of this constituent principle that it is miscible with water, in like manner as ardent spirits are, notwithstanding their inflammable nature.

"Phlogistic matters are miscible with water only in proportion as they contain a quantity of the aqueous principle in their composition. When the relative proportion of this constituent principle is less than that of the phlogiston combined with such matters, they are either immiscible with water, or miscible only in part. Thus, spirit of wine unites with water in all proportions. Ether, which is spirit of wine deprived of part of its water by means of the vitriolic acid, is not miscible with water in all proportions; but ten parts of water are requisite to the absorption of one part of ether. Oil, which has till lately water in its composition, does not in any degree mix with water. Resinous substances do not combine with water, because their aqueous part is not in sufficient quantity to serve as a medium for the union of their phlogiston. In gums the relative proportion of phlogiston is much less than in resins, and that of the water is much greater; and, by the intervention of their aqueous part, gums are readily miscible with water. Resins, when united with a due proportion of gum, are by its mediation also rendered soluble in water. But if a less proportion of the gum be joined with the resin, only a part of the compound resulting from this union is disposed to mix with water, and a residuum is left which is incapable of being dissolved in any aqueous liquor." Fixed Air. "Fixed air seems to resemble those matters which do not possess a sufficient quantity of aqueous matter to render them totally soluble in water; for after a given portion of fixed air has been imbibed by water, a residuum remains which is incapable of being absorbed by it, and is called phlogisticated air. This air may be formed from fixed air, not only by the subtraction of water, but by the addition of the inflammable principle; as when phlogiston is communicated to fixed air by electric sparks, or the vapours disengaged from a mixture of sulphur and iron filings.

"The origin of phlogisticated air, shows that its difference from fixed air consists chiefly in the deficiency of water. Hence, as metals contain no water, the phlogiston which arises from them during calcination produces no fixed but phlogisticated air. But vitriols, and all other saline matters, containing water as a constituent part, yield fixed air. Calces of metals also, which have received aqueous matter in the process of their calcination, as white-lead and other calces, which have absorbed water, together with the air, from the atmosphere, yield also fixed air. The fermentation and putrefaction of animal and vegetable substances is effected by means of their moisture, and therefore fixed air is produced in these processes. It is more effectually produced by respiration than by many other phlogistic processes, in consequence of the copious supply of the aqueous as well as the phlogistic principle, which the air receives from the lungs.

"Fixed air may be formed from vegetable acids; but when it is thus constituted, it does not differ from that which is produced from alkalies, magnesia, and other substances which yield no acid. It is therefore evident, that in each of these instances it is formed by the combination of some principles which are common to all those substances. These principles are water and phlogiston. In vegetable acids, the phlogiston combined with the water is equal in quantity to that which constitutes the inflammable part of spirit of wine; for radical or concentrated vinegar is totally inflammable. The acid state of vegetable matters is not essential to them, nor is it requisite to the production of fixed air from them; for fixed air is producible from recent plants. Hence it appears, that in their acid, as well as in their recent, vinous, or putrefactive state, they yield fixed air by means of their aqueous and phlogistic principles.

"All fixed air, from whatever subject it may be procured, or to whatever bodies it is transferred, consists constantly and invariably of the same materials, combined in the same proportions; otherwise it could not restore lime, caustic alkalies, &c., to their original mild state; because these substances cannot be recomposed but by the same proportion of their constituent principles which they contained before their decomposition. Thus, lime cannot be restored to the state of mild calcareous earth by water or by pure dephlogisticated air, because each of these principles consists only of one of the three ingredients which are requisite for that purpose. Nor can the recomposition of calcareous earth be effected by phlogisticated air, because it contains an excess of phlogiston and a defect of water. Compounds formed of such ingredients as do not contain a requisite quantity of the principles necessary to constitute fixed air, may acquire a due proportion of them by an addition of the ingredients in which they are deficient. Pure dephlogisticated air is reduced to fixed air by the communication of aqueous and phlogistic vapors disengaged from bodies by various processes; these principles have a great affinity to air, and readily combine with it. Phlogisticated air, when agitated with water, receives into its composition a quantity of aqueous particles sufficient to constitute it fixed air; and by that means it becomes capable of precipitating lime from lime-water. If the analysis and recomposition of calcareous earth be ever so often repeated, its analysis will always yield, and its recomposition will always require, the same relative quantities of air, water, and phlogiston. Fixed air therefore seems to consist of these three principles invariably and constantly combined in the same proportions. It has been frequently considered as a mere compound of air and phlogiston; but such a compound seems to approach nearer in its nature to phlogisticated air, as it is deficient in one of the principles which is essential to fixed air."

On this account of fixed air it may be observed, that notwithstanding all our author's arguments, there is no positive proof adduced against the acidity theory of fixed air. It is not certainly known whether mere phlogiston will perfectly neutralize alkalies. The colouring matter of Prussian blue indeed will certainly do so; but this is not pure phlogiston, but a compound of different substances; and besides, the alkalies neutralized by it differ very considerably from those rendered neutral by fixed air. Before we can attribute the effects of fixed air to mere phlogiston, therefore, it would be necessary to form out of a caustic alkali, by means of pure phlogiston, a salt exactly resembling mild alkali produced by the union of the same caustic salt with fixed air; so that it shall not only have the same taste and other external properties, but likewise emit fixed air upon the addition of a mineral acid. But by no experiment have we yet been able to effect this. It is absolutely necessary that the invisible and unknown substance called by Lavoisier the oxygenous principle, by others the basis of dephlogisticated or vital air, should be united to the inflammable matter, in order to the formation of the fixed air; and as this basis is likewise found to be necessary to the formation of every acid, or at least to those of the mineral kind, we have equal reason to call fixed air an acid while it displays the properties of one.

That water is an essential ingredient in the composition of fixed air as well as all other kinds of aerial vapors, is not to be doubted; but we are by no means certain whether the difference between fixed and phlogisticated air consists in the want of water in the latter. The specific gravity here cannot be any rule for us to judge of the matter: for inflammable air, the lightest of all the kinds hitherto known, cannot be produced without a certain proportion of water; and by some processes fixed air may be converted into inflammable as well as phlogisticated air.

The noxious properties of fixed air are well known, and are too often fatally experienced by the miners, facts of this who have given it the appellation of the choke-damp air on a In the Manchester Transactions, however, we have an exposed to account it. account of one who continued seven days below ground, not only exposed to the effects of this gas, but without any kind of suffocation; notwithstanding which, he was taken out alive. When first found in the pit, the sides of which had fallen in and confined him for the time above mentioned, his eyes were so swollen and protruded out of their sockets, that he had a shocking appearance; for which reason, the people tied a handkerchief round his head. While in this protruded state, however, he was capable of distinguishing objects; but in a little time his eyes sunk within their sockets, and he became quite blind. On being taken out of the pit, he seemed for some time to be in a way of recovery; but all favourable symptoms soon vanished, and he expired in three days after his release.

During all the time of his confinement he had only a space to breathe in of three yards in length and two in breadth, in which he lay upon his belly. It communicated indeed with another pit by a passage 80 yards in length and about eight or ten inches wide; but as the mouth of the pit into which he descended was flopped, and the body of earth through which he had dug thrown behind him, no circulation of air could possibly take place. The truth of this conclusion was likewise evinced by the state of the air in the other pit through which the people entered to dig out the unfortunate sufferer; for it was there so foul, that the candles they carried down with them were immediately extinguished. In this state of the air which surrounded him, it is remarkable, that the patient, who was naturally asthmatic, breathed freely, and continued to do so till his death: And on this subject Dr Percival makes the following remarks.

"As he had been long asthmatic, we may reasonably conclude from his suffering to little, that the commonly received opinion of the suffocating nature of the mephitic or choke-damp, that it destroys the elasticity of the air, and occasions a collapse of the lungs, is without foundation, notwithstanding all the respectable authorities that may be advanced in support of it. Indeed, from the phenomena which attend the extinction of life in those to whom such vapours have proved mortal, it is evident that the poison acts chiefly on the nervous system. The vital principle seems to be arrested and almost instantaneously destroyed; sometimes even without a struggle, and possibly without any antecedent pain. Pliny the elder was found, after the fatal eruption of Mount Vesuvius, exactly in the posture in which he fell, with the appearance of one asleep rather than dead. Some persons killed by foul air in a cellar at Paris, were found stiff as statues, with their eyes open, and in the posture of digging. M. Beaumé relates the history of a man who was recovered from apparent death produced by a similar cause, and who asserted that he had neither felt pain nor oppression; but that at the point of time when he was losing his senses, he experienced a delightful kind of delirium. This account receives some confirmation from what Dr Heberden says in his lectures on poisons, that he had seen an instance in which the fumes of charcoal brought on the same kind of delirium that is produced by henbane and other intoxicating poisons of the vegetable kind. Abbé Fontana breathed a certain portion of inflammable air, not only without inconvenience, but with unusual pleasure. He had a facility in dilating the breast, and never felt an equally agreeable sensation even when he inhaled the purest dephlogisticated air. But he suffered greatly in a subsequent experiment: for having filled a bladder, containing about 350 cubic inches, with inflammable air, he began to breathe it boldly after discharging the atmospheric air contained in his lungs by a violent expiration. The first inspiration produced a great oppression; towards the middle of the second, he was observed to become very pale, and objects appeared confused to his eyes: nevertheless he ventured on a third; but his strength now failed, so that he fell upon his knees, and soon afterwards upon the floor. His respiration continued to be effected with pain and difficulty, and he did not perfectly recover till the succeeding day. In this instance some degree of palsy was probably induced in the nerves of the lungs by the action of concentrated inflammable air conveyed into the vessels forcibly emptied of their atmospheric air by expiration. For, in ordinary respiration, about 35 inches of air are inhaled and exhaled; but in a violent expiration, about 60 cubic inches may be discharged. In the case of the unfortunate collier (Travis), it will be remembered, that the air was sufficiently salubrious when he went down into the pit; that by flagration it became gradually noxious; and that his nervous system must therefore have been progressively habituated to its influence. This is conformable to the observations of Dr Priestley; who found, that if a mouse can bear the first shock of being put into a vessel filled with artificial gas, or if the gas be increased by degrees, it will live a considerable time in a situation that would instantly prove fatal to other mice; and he frequently noticed, that when a number of mice had been confined in a given quantity of infected air, a fresh mouse introduced among them has immediately died in convulsions.

"It has been found by experiment, that the fumes emitted by almost every species of burning fuel prove fumes from fatal to animals, when applied in a sufficiently concentrated state. I have computed, that 300 tons of coal consumed are every day consumed in the town of Manchester during the winter season. The factitious gas generated by its consumption must amount to at least a third part of that quantity; it is probable that the smoke proceeding from it constitutes another third part; and both together are capable of occupying a space of very wide extent. Now, if it were not for the dispersion of these vapours by the wind, the precipitation of them by rain, and the influence of other causes, respiration could not be carried on in such circumstances. And we may observe, that frothy weather, which is generally serene and without wind, proves extremely oppressive, and sometimes even fatal, to asthmatic patients, especially in great cities. Indeed the rate of human mortality is nearly in proportion to their magnitude and population. It is evident, therefore, that habit, however it may abate, cannot entirely counteract the baneful effects of bad air; and those will feel its effects the more strongly, in every situation, whose nervous systems are endowed with more than ordinary sensibility. Such persons I would caution not to indulge their curiosity in the inspection of unwholesome manufactures, nor in visiting mines, caverns, caves,..." Fixed Air, gloves, hospitals, or prisons. The late Dr Brown suffered very severely by accompanying two foreigners of distinction into the duke of Bridgewater's works at Worley. It happened that they were the first who entered the tunnel on that day. The candles which they carried with them were observed to burn very dimly; but neither the passengers nor the boatmen experienced any difficulty in respiration. After remaining in the coal-pits a considerable time, they proceeded to Warrington, where Dr Brown was attacked by violent pains, which shifted suddenly from one part of his body to another; small purple spots overspread his skin; his throat became tumefied so as to render swallowing difficult; and great prostration of strength, with a low fever, ensued. The doctor was subject to the anomalous gout; had once a paralytic complaint of long continuance; and hence we may conclude that his nervous system was endowed with peculiar irritability. He was not, however, the only sufferer; for one of the foreigners was affected with similar petechiae, but attended with little pain or disorder.

Here we may observe, that our knowledge of the composition of fixed air seems to throw some light upon the subject in question. Dephlogisticated air and phlogiston are universally allowed to be the component parts; and whether we suppose, with Mr Delaval, water to be a third ingredient or not, the case will still be the same; for by whatever means the separation of the phlogiston can be effected, the air will be rendered pure. This is ascertained by some of Dr Priestley's experiments, particularly those related under the article Aerology, no 113, where the phlogiston was plainly separated by the electric spark, and the dephlogisticated air remained in a state of purity. When the atmosphere is contaminated by a mixture of fixed air, therefore, it may be purified in two very different ways: one is by the absorption of the gas without any decomposition, as by lime-water, alkaline salts, &c., the other, by the separation of its component parts, by which a portion of it must necessarily be exchanged for a portion of pure respirable air. It is probable that nature pursues both these methods in freeing the atmosphere from this noxious gas; but indeed, whatever method she takes, it is certain that any large portion of atmospheric air cannot long be contaminated with this gas, even though seemingly in a confined situation. This is evident from some experiments, where large quantities of fixed air being poured out into the atmosphere of a room, entirely vanished, so that it could not be perceived by the nicest test, in half an hour. As we are not yet acquainted with the action of atmospheric and fixed air upon each other, when the former is in large proportion, it naturally becomes a matter of suspicion whether the fixed air has not a natural tendency to decomposition, and consequently to render the air salutary after it has for a short time rendered it noxious. This is certainly the case when it meets with vegetables; for many of them are supported by every kind of noxious air. In cafes where many animals are confined together, it is probable that their death is not occasioned by the fixed air produced from their respiration, but by that which is called phlogisticated, or most probably by the total deprivation of the vital principle supplied by the de-

phlogisticated kind. The perspiration of animal bodies of itself supplies a resource for the absorption of pure fixed air; for all animal exhalations are of the alkaline kind, and therefore are capable of neutralizing this kind of gas. This is evident from a circumstance generally unnoticed, but which is obvious to every one who chooses to give himself the trouble of inquiry. In wading over the walls of rooms with lime and water, which is the first preparatory operation for painting, a violent smell of volatile alkali is perceptible; and there can be no doubt that this proceeds from a decomposition of the neutral salt formed by the union of the fixed air, produced in respiration, with the subtle phlogistic and alkaline effluvia which rise in perspiration. To this cause we may in a great measure attribute the preservation of the Russians, which Dr Percival also takes notice of, but attributes it to an accommodating quality of the human frame by which it can subsist in so great a variety of circumstances. "A Russian boor (says he), in the winter season, experiences all the varieties of air, heat, and cold, without any inconvenience. When labouring out of doors, he is exposed to the intensity of frost and snow; when he retires in the evening to his hut, which consists only of one close apartment, never ventilated during six months, he feeds upon salted fish or flesh, and afterwards repose on a greasy mattress placed over an oven in which billets of wood are burned. In this situation he is literally stewed, with his whole family, who live in a constant steam, not offensive to themselves, but scarcely supportable by a stranger. The atmosphere of a crowded town must, in many respects, resemble the foul air of a Russian cottage; yet thousands enjoy in it a considerable share of health."

On this we must further observe, that in certain cafes the human body requires much more of the vital quantity principle supplied by dephlogisticated air to support of vital air life than in others. This is particularly the case quite when much motion and exercise are used; so that the computations made of the quantity of air consumed by a human creature in a minute, must be very vague, different and variable. This was evident in M. Saufure's situation, journey to the top of mount Blanc, where, besides the general rarefaction of the atmosphere, there was a great mixture of fixed air, as appeared by the precipitation of lime-water when exposed to the action of the common air. Here, though always somewhat uneasy, yet he was comparatively well while he remained in a state of inaction; but felt excessive trouble on being obliged to exert his strength, inasmuch that he could scarce accomplish in four hours and an half, the experiments which at the foot of the mountain he would have easily done in three. "While I remained perfectly still (says he), I experienced but little uneasiness, more than a slight oppression about the heart; but on the smallest bodily exertion, or when I fixed my attention upon any object for some moments together, particularly when I pressed my chest in the act of swooping, I was obliged to rest and pant for some minutes."

From this account we must naturally conclude, that in cafes where the powers of life and circulation are strong and vigorous, a great quantity of vital principle is requisite to support life; and the sudden deprivation of any considerable part of it may occasion death, even though Fixed Air, though as much still remain as would be sufficient to support life in other circumstances. Hence, if a strong healthy animal is suddenly plunged into an atmosphere loaded with mephitic or other noxious vapours, it will very frequently die in a moment; while the mere circumstance of debility causes others to bear the same situation with impunity. In these circumstances a sudden exposure to very pure air might even be fatal to an animal very long confined in such as is noxious, just as fire may be put out by too violent a blast of air. Hence we may understand why consumptive persons are not recovered, but often made manifestly worse, by being brought into purer air; and in all probability the death of Travis was hastened by his sudden removal into the open atmosphere. His remaining alive, therefore, for such a length of time in circumstances so very unfavourable, we are to attribute in a particular manner to the extreme debility of the vital powers, by which an exceedingly small quantity of dephlogisticated air was required to support them.

We are besides to take into consideration, that in certain cases the air will impart nourishment, even to the bodies of animals as well as vegetables. Under the article Abstinence, instances are brought of animal bodies being augmented in bulk without any nourishment taken in by the mouth. In such cases we must conclude, that the nourishment came from the phlogistic particles dispersed through it. It is not impossible, therefore, that in some cases the human body, by absorbing from the atmosphere the phlogiston which it had just emitted by respiration, may purify the air which it had just before contaminated; and thus life might be prolonged in the case of Travis, who was not only defective of air proper for respiration, but of food and drink also. In cases of famine, it is manifest that there is a great absorption from the atmosphere. Thus a negro who was gibbeted at Charlestown, and had nothing given him afterwards, voided a large quantity of urine every morning; and in cases of lientery and diabetes, the quantity of evacuations greatly exceeds the nourishment taken in by the mouth. On this principle, perhaps, we may account for that very strange phenomenon of animals being found alive in the heart of solid bodies, where there could be no possibility of any connection with the external atmosphere. Instances of these are given under the article Animalcule, No. 57.

We shall conclude this part of our subject with some observations made by Mr Henry on the effects of fixed air on vegetation. Experiments on this subject had been made first by Dr Priestley, and afterwards by Dr Percival, but with very different results; the former concluding that fixed air was prejudicial to the growth of vegetables, and Dr Percival that it was favourable to it. Mr Henry endeavours to reconcile the difference. He supposes Dr Percival's meaning to be, not that fixed air, in its pure state and stagnant, was favourable to vegetables immersed in it; but that gradually applied, and in a continued stream, while the plants at the same time are not confined from the common air, do receive from fixed air such a proportion of nourishment as is sufficient for their temporary support, even when removed from every other means of being supplied with food.

Dr Priestley, in his third volume on Air, acknowledges the fairness and candour with which the experiments of Dr Percival were related, but supposes there must have been some mistake; particularly, as the air was produced by Dr Nooth's apparatus, he thinks the quantity could not be so great as was imagined. In support of this opinion he mentioned a great number of experiments, in which fixed air was tried in all proportions, from a state of purity to a mixture of one-third of fixed air with seven-eighths of common air; and in all these the fixed air was found to be injurious, and to destroy the colour of rose-leaves.

From some experiments made by Mr Henry himself, the contrary opinion seemed to be established. By these it appeared that a strawberry plant had not only been preserved alive, exposed in the middle of Dr Nooth's machine to copious streams of fixed air, from the 23rd of April to the 14th of May, but that the blossoms, which were only budded when put into the machine, had actually expanded; a strong proof that the plant had continued to vegetate. It was still alive, but in a situation somewhat drooping; and happening to be crushed on taking it out of the apparatus, it was thrown away altogether. Two sprigs of mint, with some earth loyally adhering to their roots, were subjected to a similar experiment from the 1st to the 12th of September; the one having a continual current of fixed air applied to it, but the other being left to the operation of common atmospheric air. The roots of both were cut off on the 7th: that in common air exhibited symptoms of decay on the 12th; but the other continued fresh for more than a week after the other had been decayed almost to the top.

On the 11th of April 1777, the weather being very cold and backward, Mr Henry filled the middle part of Dr Nooth's machine entirely with fixed air, by first filling it with water, inverting it in the same fluid, stopping up the capillary tubes, and then driving out the water from the vessel by a stream of fixed air from an effervescing mixture. The middle was then immediately placed in the lower part of the machine, containing an effervescing mixture also, which had been working for several minutes; and a crimson polyanthus was introduced into the middle part, and suspended by a string. In passing through the mouth of the vessel, the petals were compressed, and one or two received some damage. A young sprig of mint, with its root, was introduced the same day, and into the same vessel; while a similar sprig, as a standard, was placed in a large glass decanter. The polyanthus began to droop on the 15th, and was taken out next day shrivelled but not discoloured. The mint, when examined on the 12th, was apparently more fresh than when first put into the vessel with fixed air; the next day two young shoots appeared still more vigorous. On the 15th its appearance was more vivid than that in common air; but next day it was taken out quite dead. This sudden change, however, our author supposes to have been owing to the machine having no valve; and having been violently shaken, he suspected that some of the vitriolic acid had been forced up through the tubes; for the moisture on the inside of the middle part was found to be more acid than it should have been by fixed air alone.

The experiment was repeated on the 26th of April with a polyanthus plant with its root and flowers, which, Fixed Air, which, with several others, were put into the middle part of Nooth's machine. Here it continued till the 10th of May. The effervescence was frequently renewed; for the first four days twice, and then once a day; but the discharge of air was continually going on. It continued ten days without any signs of decay; and when taken out of the machine on the 14th day, though some of the older flowers were fading, the others were as fresh and blooming as when put into the vessel; more so than those which had been purchased on the same day, and planted in the garden. The body of the plant was green, succulent, and undecayed. The air extinguished flame. On trial it was found to be one-third fixed air; and during several days, the proportion of fixed air must have been larger.

But, when confined in vessels of fixed air, or even in Nooth's machine, with the upper part and grooved stopper put on, plants died sooner than in common air. The air measured was seven-eighths fixed air.

"I am informed (says Mr Henry), that an ingenious philosopher of Geneva has made some experiments, by which he has proved, not only that phlogiston is the food of plants, but also, to the satisfaction of Dr Priestley, that it is in the form of fixed air, in proper proportion and place, that this pabulum is administered. In regard to the animal body, it would surely be wrong to say that nothing was nutritious or salutary to it which it could not bear to receive unmixed or undiluted. Why then may we suppose, that though fixed air, when pure, may be fatal to plants confined in it, and excluded from communication with the external air; yet, when applied in a proper dose, and to plants enjoying a free intercourse with the atmosphere, it may have a contrary effect, and serve to nourish and support them? But in Dr Priestley's experiments, this free intercourse does not appear to have been allowed; and herein, I apprehend, consisted the cause of the difference in our results.

"At that time the constitution of fixed air was not understood. It is now generally allowed to be formed by a combination of phlogiston with the pure part of atmospheric air. The first of these ingredients has been proved by Dr Priestley and others to be favourable to vegetation; while plants droop and decay when exposed to the action of the latter. It should further appear from Dr Ingenhousz's experiments, that plants have the power of separating phlogiston from common air, applying it to their nurture (a), and throwing out the pure or deplogisticated residuum as excrementitious. Now allowing, what is highly probable, that they have a similar power of decomposing fixed air, and of applying and rejecting its constituent parts, our method of conducting the experiments was not injurious to the plants; whereas, when confined in close vessels, as by Dr Priestley, the plants would be suffocated in a manner revered to what would happen to an animal. For as in that case, from a want of communication with the atmosphere, necessary to carry off the phlogistic air from the lungs (according to the beautiful theory of respiration advanced and so well supported by Dr Priestley), the animal must perish; so, in the other instance, the plant would die if cut off from the air of the atmosphere in such a manner that the pure air extracted by its vessels would not be conveyed from it. Fixed Air. For in these circumstances, this fluid, so salutary to animal but destructive to vegetable life, must be accumulated in the body of the plant; and its functions being thus impeded, death is the necessary consequence."

As fixed air is now an article of the materia medica, a method of obtaining it readily and in large quantity becomes an object of considerable consequence. Mr Henry, who has proved that fixed air is the proper fixed air, safe of ferments, and the immediate cause of fermentation, describes an apparatus for impregnating wort or other fermentable liquors with it. This apparatus is represented Plate CXCIII.

AA, Fig. 1 represents the cask in which the wort is to be impregnated; dd, the flings by which the air-vessel is to be let down.

Fig. 2. DD, The air-vessel, similar to the bottom part of Dr Nooth's glass machine, to be made of glass or earthen ware. cc, A glass-flopper ground in to fit the mouth of the vessel, having a number of capillary tubes running from bottom to top in a diverging direction, so as to spread the air in its passage through the liquor.

Fig. 3. The stopper viewed separately to show the capillary tubes.

The method of using this apparatus is obvious from an inspection of the figure; but at the same time it must be equally evident, that it cannot be applied where any very large quantity is to be impregnated. Where great quantities of fixed air are required, we must also use great quantities of fermenting materials; and it would be inconvenient in the highest degree to immerse these in the liquor to be fermented; not to mention, that where large quantities of this kind of materials are mixed, they ought frequently to be stirred or shaken, lest they should concreted into hard lumps; while at the same time they are often apt to swell, and would thus endanger the spoiling of the liquor altogether. It must also be remarked, that any liquid receives an impregnation of fixed air more readily from the surface than by blowing it through the mass of liquid. The apparatus represented fig. 4, therefore seems preferable to that of Mr Henry, as capable of being extended indefinitely almost without any additional trouble. ABCD represents a large wooden cask filled with materials to the height represented by kk. E is a large flat cooler for holding the liquor to be impregnated. This vessel is to be closely covered, and may be conveniently made of lead, having a wooden top, the edges of which are closely luted all round with a mixture of salad oil and finely powdered chalk. f, Represents a tin pipe, about an inch in diameter, by which a communication is made between the cask and cooler for the transmission of the fixed air. g b, Is a wooden axis passing quite through the cask from top to bottom, and moveable on a centre b, having a strong handle at top, to turn it in order to stir the mixture. iii, Are four crofs blades fixed into the axis, which, in consequence of turning the handle, stir and raise a great commotion in the liquor contained in the cask. m, Is a large hole stopped with a wooden plug, by which the materials may be put in or taken out; and for this last purpose a kind of ladle with an upright stem

(a) See this further discussed under the article Aerology, n° 51, 52. Fixed Stars item as M may be made use of. I. Is a plug flopping up a hole in the lid of the cooler E, by which the liquor to be impregnated may be put in, and let out again by the cock n.

In this apparatus it is evident, that when an effervescing mixture is put into the cask, the fixed air must pass through the tube f into the cooler, where it will be absorbed by the liquor as fast as it is emitted by the materials; but in order to prevent it from escaping, all the junctures must be luted carefully with the mixture of salad oil and chalk already mentioned, which is both sufficiently adhesive, and remaining soft for a long time, may be instantaneously repaired where it happens to be broken. When the effervescence begins to turn languid, it may instantly be quickened by turning the handle; but this will disturb the luting at o2, which must therefore be clapped close all round the axis as soon as the matter is sufficiently stirred.

Fixed Stars, are such as constantly retain the same position and distance with respect to each other*; by which they are contradistinguished from erratic or wandering stars, which are continually shifting their situation and distance.—The fixed stars are what we properly and absolutely call stars: the rest have their peculiar denominations of planet and comet. See Astronomy, per Index.