in a relative sense, generally signifies that sensation which accompanies the transition of the fine vessels of the human body from an expanded to a more contracted state. In an absolute sense, it means the agent by which that sensation is produced.
Concerning the cause of this sensation, which alone can be properly called cold in the abstract, philosophers are by no means agreed; some maintaining that the term is merely relative, and owing only to the different degrees of heat contained in different bodies; others, that cold is as really and truly a substance as heat itself. The arguments on both sides may be stated as follows:
1. The sensation of cold may be produced without abstracting from the human body any part of its sensible heat; nay, from some observations made by the thermometer, it appears, that a violent degree of cold may be felt when the body is really hotter than usual. Examples of this occur in the cold fits of intermittents, and the chills which usually precede fevers of all kinds.
2. The body may be so deprived, that the same substance may feel hot and cold to different parts of it. Thus, suppose a person has one hand cooled so as to think Fahrenheit's thermometer to 60°, while the other raises it to 90°; if both of them are now plunged into a vessel of water reduced to the temperature of 75°, this water will feel hot to one hand and cold to the other.
3. Though we see a visible spring and fountain of heat, heat, viz. the sun, who by his light diffuses warmth to the whole world, yet we perceive no such spring or origin of cold. The sun's presence always brings warmth; and of this warmth there is a very evident cause, viz. the light which is continually flowing from him: his absence is as certainly attended with cold; but for this cold we observe no positive cause. We see, that to produce the most intense cold nothing else than the mere privation of the sun's light is requisite. As therefore we have already seen that it is possible to produce different sensations of heat and cold, without varying the temperature of the substance by which they are produced, it seems most probable, that these sensations are the result of a mere negation, or want of that heat to which our bodies have been accustomed, or which is necessary to their subsistence.
In answer to the above, it hath been urged,
1. The two first instances prove nothing. From them it only appears, that cold produces a contraction of the fine vessels of our bodies; and that, by whatever cause this contraction is produced, the same sensation always follows it: but whether this sensation is produced in all cases by a negative or a positive, can by no means be determined by such experiments.
2. The third argument proves by far too much: for if the mere absence of the sun's light and heat was sufficient to produce cold, then caves and other places that are never exposed to the sun, ought to be cold beyond all imagination. The contrary, however, is manifest to daily experience. At a certain depth below ground, an uniform temperature is observed, which scarcely ever varies, provided a circulation of air is prevented as much as possible. In some caverns indeed, such as those in Mount Athos, where the air circulates freely, the cold is very intense*. It would seem therefore, that as the sun is the origin and fountain of heat, so is our atmosphere the source from whence cold is derived; and that this atmosphere, or something in it, is as really and positively cold, as the light of the sun is really and positively heat. This will become the more probable when we consider, that the beams of the sun are not capable, as emitted from him, of heating the atmosphere. They must first be detained in it by reflection from the earth; and where this reflection cannot reach, as on the tops of very high mountains, an intense degree of cold is always found to take place. Again, even in those places where the cold is most intense, it is found only to affect the surface of the ground. This shows, that the cause of cold lies in the atmosphere, and not in the absence of the sun's light; otherwise, in those regions, however cold the surface was, the under parts of the soil must be much colder. We see, therefore, that there is in nature a positive source of cold as well as of heat; the atmosphere being the source of the one, as the sun is of the other; and these two always acting opposite to one another, contribute to preserve that exact balance which is necessary for the existence of the system of nature in the form we see it; and, were the one or the other finally to get the better, the whole world, or at least its inhabitants, must perish in a miserable manner.
It will scarce be denied, that, were the sun to cease the emission of his light for a few months, the most intense and violent cold would by that time have taken place in the atmosphere. But, what has become of the immense quantity of heat and light emitted from the sun since the creation of the world? It is absorbed by terrestrial bodies, we know; but what becomes of it afterwards? The atmosphere perhaps takes it from terrestrial bodies: nay, indeed, there is here not the least doubt; for we know that exposure to the air is a very ready way to cool any thing. Still, however, the difficulty is not solved: for it is most certain, that, since the world was created, the earth and its atmosphere have received as much heat from the sun as would have been sufficient to have set them on fire a thousand times over; and seeing this hath not been done, we must necessarily conclude, either that heat, after some time, ceases to be heat, or that there is in nature a power able to countermand or suspend its effects; and that this power, though invisible as heat itself is, must be really and truly a positive substance, and is that which we commonly call cold.
The first of these positions, viz. that heat, after some time, ceases to be heat, is contrary to all experience. It is certain, that no body whatever loses its heat but by communicating it to others. Were there not, therefore, an opposite principle in nature, the quantity of heat accumulated in terrestrial bodies must have been continually increasing; and, as already observed, would long since have destroyed the whole world. By supposing, however, a cold principle as well as a hot one, the difficulty entirely vanishes. On this supposition it is impossible that a quantity of heat can enter any substance without expelling an equal quantity of cold from it. When the cause producing the heat is removed, the cold then re-enters and displaces the heat, and so on.
3. That cold is a positive principle is not a mere Experiment, but may be ascertained by experiment. From what we have just now said it follows, that if a body is heated, the cold ought to fly from it; and, attacking those substances in the vicinity of the heated body, make them colder than before. To this purpose we have an experiment recorded in the Philosophical Transactions, no 274, p. 951. It was made by M. Geoffroy. "I put (says he) some cold water into a great basin. I put into the middle of the water a cucurbit of glass full of water equally cold. I put into the cucurbit a very good thermometer, which I let lie a good while for a trial. When it was adjusted to a degree proportionable to the cold of the water, I threw suddenly into the water in the basin four or five shovels full of coals well lighted; and in an instant the liquor of the thermometer descended two or three lines. After some moments the liquor rose again, when the heat in the water of the basin was communicated to the glass."
These are the principal arguments that have been urged pro and con in this question. The last mentioned experiment, if it can be depended on, seems to be decisive in favours of those who assert cold to be a positive substance. But however philosophers may decide this question, we believe the generality of mankind will always take it for granted that cold is a substance as well as heat.
Among those who allow cold to be a substance, there there is no small disagreement about what sort of substance it is. Some have supposed it to consist in certain nitrous or saline particles diffused through the atmosphere; others attribute cold to the action of the electric fluid. The first hypothesis is evidently false; for if such saline particles existed, it is impossible but they must on some occasions discover themselves, which they are never known to do. Concerning the latter, no experiments either have been or probably can be made; because every experiment in electricity we can make is only throwing the fluid into some kind of motion, in which case it would be more apt to produce heat than cold. It is observable, however, that the readiest conductors of the electric fluid are likewise the best conductors of heat and cold; and vice versa. Thus, metals, which are the best conductors of electricity, also transmit heat or cold very readily through them; whereas wool, hair, silk, &c., which will not conduct this fluid, are found to be the best preservatives against excesses of heat or cold. See Electricity.
Most kinds of saline substances mixed with water make the mixture considerably colder than either the salt or the water was before. In the paper quoted above, Mr Geoffroy gives an account of several experiments with regard to this power of the salts to produce cold. Four ounces of sal ammoniac, mixed with a pint of common water, made his thermometer descend two inches and nine lines below the temperature of the water, in less than a quarter of an hour. An ounce of the same salt, put into four or five ounces of distilled vinegar, made the liquor descend two inches and three lines. 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 that of nitre, it sunk three inches and six lines; and what is very remarkable, though this mixture was so extremely cold, the vapours raised by it had a considerable degree of heat.
Four ounces of saltpetre mixed with a pint of water, sunk the thermometer one inch three lines; but a like quantity of sea-salt sunk it only two lines. With acids, even with its own spirit, sea-salt produced a considerable heat. Volatile alkaline salts produced cold in proportion to their purity; but fixed alkalies, heat. The greatest degree of cold, however, as yet produced by mixtures of this kind, was shown by Mr Homburg; and the experiment may be performed as follows. "Take a pound of corrosive sublimate, and as much sal ammoniac; powder them separately, and mix both the powders very exactly; put the mixture into a vial, pouring upon it a pint and a half of distilled vinegar, and shake all well together. This composition grows so cold that a man can scarcely hold it in his hands in summer. And it happened, as Mr Homburg was making the experiment, that the subject froze." This also once happened to M. Geoffroy with common water and sal ammoniac; but he says he never could make the experiment succeed again.
If instead of making these experiments with water in its fluid state, we take it when converted into ice or rather snow, incredible degrees of cold may be produced. A mixture of snow and common salt sinks Fahrenheit's thermometer to 0°. If pot-ashes are mixed with powdered ice, it sinks 8° farther. Two affusions of spirit of salt on pounded ice sunk it 14° below 0°; but by repeated affusions of spirit of nitre, Mr Fahrenheit sunk it to 40° below 0°; and the spirit of nitre itself though very strong was then frozen.
Even these excessive degrees of cold are naturally produced by the atmosphere in some parts of the world. In 1703 Mr Derham acquaints us that his thermometer was within one tenth of an inch of its station when plunged into a mixture of snow and salt. In 1732, the thermometer at Peterburgh stood at 28° below 0°. In 1737, when the French academicians wintered at the north polar circle or near it, the thermometer sunk 33° below 0°. The spirit of wine with which some of their thermometers were filled was then frozen; the air, when suddenly admitted into their warm rooms, became intolerable to their bodies, their breasts seemed to be rent when they breathed it, and the moisture of the air was immediately converted into whirls of snow. In this degree of cold every metallic substance, when touched, blistered the skin like red-hot iron. This, however, is but trifling in comparison of what hath been observed at a place in Siberia, lying in Lat. 58° 10', where the thermometer fell to 118° below 0°; nay, by some late accounts from that country, we are informed, that the cold hath been observed there of such a strength as to freeze the quicksilver in the thermometers. When Mr Brown first made this famous experiment of freezing quicksilver, the thermometer at Peterburgh stood at 40° below 0°. By repeated affusions of spirit of nitre on snow, he made it sink 168 degrees lower; after which the cold became immeasurable, because the quicksilver was frozen. But for a particular account of this experiment, see the article Congelation. Very great degrees of cold also may be produced by evaporation; for a particular account and explanation of which, see the article Evaporation.
medicine. See (the Index subjoined to) Medicine.
farriery. See there, § iii.