in a relative sense, signifies the sensation which accompanies a transition of the fine vessels of the human body from an expanded to a more contracted state. In an absolute sense, it signifies the cause of this transition; or, in general, the cause of the contraction of every substance, whether solid or fluid, in nature.

The arguments concerning the substance of cold tends the abstract, are discussed under the article Chemistry, to make both which we must at present refer the reader. In that article it is observed, that cold naturally tends to make bodies electric which are not so naturally, and to increase the electric properties of those which are; and in confirmation of the hypothesis there advanced, it may be observed, that all bodies do not transmit cold equally well; but that the best conductors of electricity, viz., metals, are likewise the best conductors of cold. We Bodily remedies may also add, that when the cold has been carried to such an extremity as to render any body an electric conductor, it then ceases to conduct the cold as well as formerly. This is exemplified in the practice of the Laplanders, who, in winter, carry pieces of ice, which in the winter time must always be electric in these countries, and put them into their windows; which they find to be much more effectual in keeping out the cold than any other substance.

Cold, as well as heat, may be produced artificially, why cold though we have no method of making cold increase itself as heat will do. The reason of this easily appears from what is said on the subject of cold under the article Chemistry: for if this consists in a partial cessation of motion in the elementary fluid, it is plain, that though we may partly put an end to this motion in a very small part of it; yet that of the surrounding atmosphere extending for an immense way farther than we can extend our influence, will quickly counteract our operations, and reduce the bodies to the same temperature they were of before. Though there are therefore some liquids which by mixture will produce considerable degrees of cold; yet by being left to the action of the surrounding warm atmosphere, the heat is quickly communicated from it to them, and the effect of the mixture ceases. The case is very different with heat; for this fluid, of itself naturally very much inclined to motion, no sooner finds an opportunity of exerting its action, than vast quantities of what was formerly at rest rush from all quarters to the place where The power of producing cold belongs particularly to bodies of the saline clats. In a paper of the Philosophical Transactions, No. 274, Mr. Geoffroy gives an account of some remarkable experiments with regard to the production of cold. Four ounces of sal ammoniac dissolved in a pint of water, made his thermometer descend two inches and three quarters in less than fifteen minutes. An ounce of the same salt put into four or five ounces of distilled water, made the thermometer descend two inches and a quarter. Half an ounce of sal ammoniac mixed with three ounces of spirit of nitre, made the thermometer descend two inches and five lines; but on using spirit of vitriol instead of nitre, it sunk two inches and six lines. In this last experiment it was remarked, that the vapors raised from the mixture had a considerable degree of heat, though the liquid itself was so extremely cold. Four ounces of saltpetre mixed with a pint of water, sunk the thermometer one inch three lines; but a like quantity of sal-falt sunk it only two lines. Acids always produced heat, even common salt with its own spirit. Volatile alkaline salts produced cold in proportion to their purity, but fixed alkalies heat.

The greatest degree of cold produced by the mixture of salts and aqueous fluids was that shown by M. Homburg; who gives the following receipt for making the experiment: "Take a pound of corrosive sublimate, and as much sal ammoniac; powder them separately, and mix the powders very exactly; put the mixture into a vial, pouring upon it a pint and a half of distilled vinegar, shaking all well together." This composition grows so cold, that a man can scarcely hold it in his hands in summer; and it happened, as M. Homburg was making the experiment, that the subject froze. The same thing once happened to M. Geoffroy in making an experiment with sal ammoniac and water, but it never was in his power to make it succeed a second time.

If, instead of making these experiments, however, with fluid water, we take it in its congealed state of ice, or rather snow, degrees of cold will be produced vastly superior to any we have yet mentioned. A mixture of snow and common salt sinks Fahrenheit's thermometer to 0°; potashes and powdered ice sink it eight degrees farther; two affusions of spirit of salt on pounded ice sunk it below 14½° below 0°; but by repeated affusions of spirit of nitre Mr. Fahrenheit sunk it to 40° below 0°. This is the ultimate degree of cold which the mercurial thermometer will measure: because the mercury itself begins then to congeal; and therefore we must afterwards have recourse to spirit of wine, naphtha, or some other fluid which will not congeal. The greatest degree of cold hitherto producible by artificial means has been 80° below 0°; which was done at Hudson's Bay by means of snow and vitriolic acid, the thermometer standing naturally at 20° below 0°. Greater degrees of cold than this have indeed been supposed. Mr. Martin, in his Treatise on Heat, relates, that at Kirenga in Siberia, the mercurial thermometer sunk to 118° below 0°; and Professor Brown at Petersburg, when he made the first experiment of congealing quicksilver, fixed the point of congelation at 350° below 0°; but Dr. Black, as soon as the experiment was made known in this country, observed, that in all probability the point of congelation was far above this. His reasons for supposing this to be the case were, that the mercury descended regularly only to a certain point, after which it would descend suddenly and by starts 100 degrees at a time. This, he conjectured, might proceed from the irregular contraction of the metal after it was congealed; and he observed, that there was one thermometer employed in the experiment which was not frozen, and which did not descend so low by a great many degrees. Experience has since verified his conjecture; and it is now generally known, that 40° below 0° is the freezing point of quicksilver.

Since the discovery of the possibility of producing cold by artificial means, various experiments have been made on the efficacy of saline substances in this way; all of which, when properly applied, are found to have a considerable degree of power. Dr. Boerhaave found, that both sal ammoniac and nitre, when well dried in a crucible, and reduced to fine powder, will produce a greater degree of cold than if they had not been treated in this manner. His experiments were repeated by Mr. Walker, apothecary to the Redcliffe Infirmary in Oxford with the same result: but he found, that his thermometer sunk 32° by means of a solution of sal ammoniac; when Boerhaave's, with the same, fell only 28°. Nitre sunk it 19°. On mixing the two salts together, he found that the power of producing cold was considerably increased. By equal parts of these salts, he cooled some water to 22°; the thermometer standing at 47° in the open air. Adding to this some powder of the same kind, and immersing two small phials in the mixture, one containing boiled and the other unboiled water, he soon found them both frozen, the unboiled water freezing first.

Having observed that Glauber's salt, when it retains the water of crystallization, produces cold during its solution, he tried its power when mixed with the other salts, and thus sunk the thermometer from 65° to 19°; and thus he was able to freeze water when the thermometer stood as high as 70°. And, lastly, by first cooling the salts in water in one mixture, and then making another of the cooled thermometers, he was able to sink the thermometer 64°. Thus he froze a mixture of spirit of wine and water in the proportion of seven of the latter to one of the former; and by adding a quantity of cooled materials to the mixture in which this was frozen, the thermometer sunk to —4°, or 69°. Spirit of nitre diluted with water reduced the thermometer to —3°; and, by the addition of sal ammoniac, to —15°. Nitrous ammoniac reduced it from 50° to 15°; but the cold was not augmented by the addition of sal ammoniac or nitre.

The most remarkable experiment, however, was Mr. Walker's method with spirit of nitre poured on Glauber's salt, the effect of freezing feet of which was found to be similar to that of the quicksilver, same spirit poured on ice or snow; and the addition of sal ammoniac rendered the cold still more intense. The proportions of these ingredients recommended by Mr. Walker are concentrated nitrous acid two parts by weight, water one part; of this mixture cooled to the temperature of the atmosphere (18 ounces of Glauber's salt a pound and an half avoirdupois, and of sal ammoniac 12 ounces. On adding the Glauber's salt to... the nitrous acid, the thermometer fell from 50° to —1°, or 52 degrees; and on the addition of the sal ammoniac, to —9°. Thus Mr Walker was able to freeze quicksilver without either ice or snow, when the thermometer stood at 45°. For the experiment four pans were procured of different sizes, so that one might be put within the other. The largest of these pans was placed in a vessel till larger, in which the materials for the second frigorific mixture were thinly spread in order to be cooled; the second pan, containing the liquor, viz. the vitriolic acid properly diluted, was placed in the largest pan; the third pan, containing the salts for the third mixture, was immersed in the liquor of the second pan; and the liquor for the third mixture was put into wide-mouthed phials, which were immersed in the second pan likewise, and floated round the third pan; the fourth pan, which was the smallest of all, containing its cooling materials, was placed in the midst of the salts of the third pan. The materials for the first and second mixtures consisted of diluted vitriolic acid and Glauber's salt; the third and fourth of diluted nitrous acid, Glauber's salt, and sal ammoniac, in the proportions above mentioned. The pans being adjusted in the manner already mentioned, the materials of the first and largest pan were mixed; this reduced the thermometer to 10°, and cooled the liquor in the second pan to 20°; and the salts for the second mixture, which were placed underneath in the large vessel, nearly as much. The second mixture was then made with the materials thus cooled, and the thermometer was reduced to 3°. The ingredients of the third mixture, by immersion in this, were cooled to 10°; and, when mixed, reduced the thermometer to —15°. The materials for the fourth mixture were cooled by immersion in this third mixture to about —12°. On mixture they sunk the mercury very rapidly, and seemingly below —40°, though the froth occasioned by the ebullition of the materials prevented any accurate observation. The reason why this last mixture reduced the thermometer more than the third, though both were of the same materials, and the latter of a lower temperature, was supposed to have been partly because the fourth pan had not another immersed in it to give it heat, and partly because the materials were reduced to a finer powder.

The experiments were repeated with many variations; but only one mixture appeared to Dr Beddoes, by whom the account was communicated to the Royal Society, to be applicable to any useful purpose. This is oil of vitriol diluted with about an equal quantity of water; which, by dissolving Glauber's salt, produces about 46° of cold, and by the addition of sal ammoniac becomes more intense by a few degrees. At one time, when Mr Walker was trying a mixture of two parts of oil of vitriol and one of water, he perceived, that at the temperature of 35° the mixture coagulated as if frozen, and the thermometer became stationary; but on adding more Glauber's salt, it fell again in a short time: but less cold was produced than when this circumstance did not occur, and when the acid was weaker. The same appearance of coagulation took place with other proportions of acid and water, and with other temperatures.

It is observable, that this effect of Glauber's salt in producing cold took place only when it was possessed of its water of crystallization; and thus the mineral alkali also augmented the cold of some of the mixtures: but when the water of crystallization was dissipated, neither of them had any effect of this kind.

"This circumstance (says Dr Beddoes) leads us in experiment, some measure to the theory of the phenomenon. Water undoubtedly exists in a solid state in crystals; it must therefore, as in other cases, absorb a determinate quantity of fire before it can return to its liquid state. On this depends the difference between Glauber's salt and mineral alkali in its different states of crystallization and efflorescence. The same circumstance, too, enables us to understand the great effect of Glauber's salt; which, as far as I understand, has the greatest quantity of water of crystallization."

On this the reviewers remark, that "if in summer the water brought from a deep well is at 52°, in this cheap and easy way it might be reduced to 12°; and wine placed in it would be chilled."

These excessive degrees of cold occur naturally in many parts of the globe in the winter-time. It is true, we are very much unacquainted with them in this country: yet in the winter of 1780, Mr Wilson of Glasgow observed, that a thermometer laid on the snow sunk to 25° below 0; but this was only for a short time; and in general our atmosphere does not admit of very great degrees of cold for any length of time. Mr Derham, however, in the year 1708, observed in England, that the mercury stood within one-tenth of an inch of its station when plunged into a mixture of snow and salt. In 1732, the thermometer at Petersburg stood at 28° below 0; and in 1737, when the French academicians wintered at the north polar circle, or near it, the thermometer sunk to 33° below 0; and in the Asiatic and American continents, still greater degrees of cold are very common.

The effects of these extreme degrees of cold are very surprising. Trees are buried, rocks rent, and rivers and lakes frozen, several feet deep: metallic substances blister the skin like red-hot iron; the air, when drawn in by respiration, hurts the lungs, and excites a cough; even the effects of fire in a great measure seem to cease; and it is observed, that though metals are kept for a considerable time before a strong fire, they will still freeze water when thrown upon them. When the French mathematicians wintered at Tornea in Lapland, the external air, when suddenly admitted into their rooms, converted the moisture of the air into whirls of snow; their breasts seemed to be rent when they breathed it, and the contact of it was intolerable to their bodies; and the spirit of wine, which had not been highly rectified, burnt some of their thermometers by the congelation of the aqueous part.

Extreme cold very often proves fatal to animals in those countries where the winters are very severe; and thus 7000 Swedes perished at once in attempting to pass the mountains which divide Norway from Sweden. It is not necessary, indeed, that the cold, in order to prove fatal to human life, should be very intense as has been just mentioned. There is only requisite a degree somewhat below 32° of Fahrenheit, accompanied with snow or hail, from which shelter cannot be obtained. The snow which falls upon the clothes, or the uncovered parts of the body, then melts, and by a continual evaporation carries off the animal heat to such such a degree, that a sufficient quantity is not left for the support of life. In such cases, the person first feels himself extremely chill and uneasy; he begins to turn listless, unwilling to walk or use exercise to keep himself warm; and at last turns drowsy, fits down to refresh himself with sleep, but wakes no more. An instance of this was seen not many years ago at Terra del Fuego; where Dr Solander, with some others, having taken an excursion up the country, the cold was so intense, that one of their number died. The Doctor himself, though he had warned his companions of the danger of sleeping in that situation, yet could not be prevented from making that dangerous experiment himself; and though he was awakened with all possible expedition, his body was so much shrunk in bulk, that his shoes fell off his feet, and it was with the utmost difficulty that he was recovered.

In those parts of the world where vast masses of ice are produced, the accumulation of it, by absorbing the heat of the atmosphere, occasions an absolute sterility in the adjacent countries, as is particularly the case with the island of Iceland; where the vast collections of ice floating out from the Northern Ocean, and stopped on that coast, are sometimes several years in thawing. Indeed, where great quantities of ice are collected, it would seem to have a power like fire, both augmenting its own cold and that of the adjacent bodies. An instance of this is related under the article Evaporation, in Mr Wedgwood's experiment, where the true cause of this phenomenon is also pointed out.

medicine. See Medicine Index.

farriery. See there, § iii.