COLD

, the privation of heat, or the opposite to it.

As it is supposed that heat consists in a particular motion of the parts of the hot body, hence the nature of cold, which is its opposite, is deduced; for it is found that cold extinguishes, or rather abates heat; hence it would seem to follow, that those bodies are cold, which check and restrain the motion of the particles in which heat consists.

In general, cold contracts most bodies, and heat expands them: though there are some instances to the contrary, especially in the extreme cases or states of these qualities of bodies. Thus, though iron, in common with other bodies, expand with heat, yet, when melted, it is always found to expand in cooling again. So also, though water always is found to expand gradually as it is heated, and to contract as it cools, yet in the act of freezing, it suddenly expands again, and that with a most enormous force, capable of rending rocks, or bursting the very thick shells of metal, &c. A computation of the force of freezing water has been made by the Florentine Academicians, from the bursting of a very strong brass globe or shell, by freezing water in it; when, from the known thickness and tenacity of the metal, it was found that the expansive power of a spherule of water only one inch in diameter, was sufficient to overcome a resistance of more than 27,000 pounds, or 13 tons and a half. See also experiments on bursting thick iron bomb-shells by freezing water in them by Major Edward Williams of the Royal Artillery, in the Edinb. Philos. Trans. vol. 2.

Such a prodigious power of expansion, almost double that of the most powerful steam engines, and exerted in so small a mass, seemingly by the force of cold, was thought a very powerful argument in favour of those who supposed that cold, like heat, is a positive substance. Dr. Black's discovery of latent heat, however, has now afforded a very easy and natural explication of this phenomenon. He has shewn, that, in the act of congelation, water is not cooled more than it was before, but rather grows warmer: that as much heat is discharged, and passes from a latent to a sensible state, as, had it been applied to water in its fluid state, would have heated it to 135°. In this process, the expansion is occasioned by a great number of minute bubbles suddenly produced. Formerly these were supposed to be cold in the abstract; and to be so subtle, that, insinuating themselves into the substances of the fluid, they augmented its bulk, at the same time that, by impeding the motion of its particles upon each other, they changed it from a fluid to a solid. But Dr. Black shews that these are only air extricated during the congelation; and to the extrication of this air he ascribes the prodigious expansive force exerted by freezing water. The only question therefore now remaining, is, By what means this air comes to be extricated, and to take up more room than it naturally does in the fluid. To this it may be answered, that perhaps part of the heat which is discharged from the freezing water, combines with the air in its unelastic state, and, by restoring its elasticity, gives it that extraordinary force, as is seen also in the case of air suddenly extricated in the explosion of gun-powder.

Cold also usually tends to make bodies electric, which are not so naturally, and to increase the electric properties of such as are so. And it is farther found that all substances do not transmit cold equally well; but that the best conductors of electricity, viz metals, are likewise the best conductors of cold. It may farther be added, that when the cold has been carried to | such an extremity as to render any body an electric, it then ceases to conduct the cold so well as before. This is exemplified in the practice of the Laplanders and Siberians; where, to exclude the extreme cold of the winters from their habitations the more effectually, and yet to admit a little light, they cut pieces of ice, which in the winter time must always be electric in those 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 is the destroyer of all vegetable life, when increased to an excessive degree. It is found that many garden plants and flowers, which seem to be very stout and hardy, go off at a little increase of cold beyond the ordinary standard. And in severe winters, nature has provided the best natural defence for the corn fields and gardens, namely, a covering of snow, which preserves such parts green and healthy as are under it, while such as are uncovered by it are either killed or greatly injured.

Dr. Clarke is of opinion, that cold is owing to certain nitrous, and other saline particles, endued with particular figures proper to produce such effects. Hence, sal-ammoniac, saltpetre, or salt of urine, and many other volatile and alkalizate salts, mixed with water, very much increase its degree of cold. In the Philos. Trans. number 274, M. Geoffroy relates 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 2 inches and 3/4 in less than 15 minutes. An ounce of the same salt put into 4 or 5 ounces of distilled water, made the thermometer descend 2 inches and 1/4. Half an ounce of sal-ammoniac mixed with 3 ounces of spirit of nitre, made the thermometer descend 2 inches and 5/12; but, on using spirit of vitriol instead of nitre, it sunk 2 inches and 1/2. In this last experiment it was remarked, that the vapours 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 an inch and 1/4; but a like quantity of sea salt sunk it only 1/6 of an inch. 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.

But the greatest degree of cold produced by the mixture of salts and aqueous fluids, was that shewn by Homberg; 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 well; 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 it can scarce be held in the hand in summer; and it happened, as M. Homberg was making the experiment, that the matter 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 with fluid water, it be taken in its congealed state of ice, or rather snow, degrees of cold will be produced greatly superior to any that have yet been mentioned. A mixture of snow and common salt sinks Fahrenheit's thermometer to 0; pot ashes and pounded ice sunk it 8 degrees far- ther; two affusions of spirit of salt on pounded ice sunk it 14 1/2 below 0; and by repeated assusions of spirit of nitre M. Fahrenheit sunk it to 40° below 0. This is the ultimate degree of cold which the mercurial thermometer will measure; for the mercury itself begins then to congeal; and therefore recourse must afterwards be had to spirit of wine, naptha, or some other fluid that will not congeal. The greatest degree of cold hitherto produced 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. Indeed greater degrees of cold than this have 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 from later experiments it has been more accurately determined, that 40° below 0 is the freezing point of quicksilver.

The most remarkable experiment however was made by Mr. Walker of Oxford, with spirit of nitre poured on Glauber's salt, the effect of which was found to be similar to that of the 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 a half avoirdupois; and of sal-ammoniac, 12 ounces. On adding the Glauber's salt to the nitrous acid, the thermometer fell 52°, viz from 50 to - 2; and on the addition of the salammoniac, it fell to - 9°. Thus Mr. Walker was able to freeze quicksilver without either ice or snow, when the thermometer stood at 45°; viz, by putting the ingredients in 4 different pans, and inclosing these within each other.

Excessive degrees of cold occur naturally in many parts of the globe in the winter season.

Although the thermometer in this country hardly ever descends so low as 0, yet in the winter of 1780, Mr, Wilson of Glasgow observed, that a thermometer laid on the snow sunk to 25° below 0; and Mr. Derham, 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. At Petersburg, in 1732, the thermometer stood at 28° below 0; and when the French academicians wintered near the polar circle, the thermometer sunk to 33° below 0; and in the Asiatic and American continents, still greater degrees of cold are often observed.

The effects of these extreme degrees of cold are very surprising. Trees are burst, 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 breathing, 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, burst some of their thermometers by the congelation of the aqueous part.

Extreme cold too often proves fatal to animals in those countries where the winters are very severe; thus 7000 Swedes perished at once in attempting to pass the mountains which divide Norway from Sweden. But it is not necessary that the cold, in order to prove fatal to human life, should be so very intense as has just been mentioned; it is only requisite to be a little below 32° of Fahrenheit, or the freezing point, 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 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 turns listless, unwilling to walk or use exercise to keep himself warm, and at last turns drowsy, sits down to refresh himself with sleep, but wakes no more.