of heat, and the point at which they become fluid is still a settled one. This point is called their melting point, which in the undermentioned substances is as follows: 594° Milk Lead 30° Oil of bergamot 23 20 133 Oil of turpentine 14 92 Mercury 50 Liquid ammonia 46 36 Ether 4.6 In general those solid bodies which crystallize have no interval between solidity and fluidity, while those which do not assume the crystallized form exhibit that gradual softening which I just now remarked. Some bodies, after having appeared in a fluid state by means of heat, concrete again into the same form as before they were melted, as salt, ice, the metals, especially gold and silver; the melting of these bodies is called fusion. There are other bodies, as earths, stones, oxides of metals, which suffer alteration and concrete into masses like glass, of a hard and brittle nature, and whose particles, when broken, have hard and polished surfaces. The making of this glassy concretion is called vitrification. Fluidity has hitherto been considered as de pending upon the quantity of sensible or obvious heat; but though it may render the subject rather complex, we shall find there is something else to be attended to. From facts and experiments it is proved that fluidity does not depend upon the sensible heat, or that which we perceive in bodies by our senses, and by the thermometer, but upon a certain quantity of what Dr. Black called latent heat, imperceptible to us, but ready to emerge on proper occasions and assume a sensible form. I mean by obvious or sensible heat, that which is so far in a fluctuating state, that if you apply any body which contains and excess of it to a cooler one, it immediately leaves the hotter, and flies to the cooler body, so as to restore an equilibrium. This will be the case of all bodies which are conductors of caloric or heat, and in proportion to their conducting power, while they do not change their state; but when they are about to change, their temperature or sensible heat is not increased, for the caloric is absorbed, and instead of being sensible, serves to render them fluid. Thus ice conducts caloric at any degree below the freezing point, and a sensible increase of the temperature may be observed in it till it reaches that point, viz. 32°; but when it has attained that degree it will remain at or near it till the whole is melted. The reason is clear, that when arrived at this point, the body is in a state for the absorption of caloric, which process goes on without any increase of temperature till the whole is rendered fluid. In proof I shall instance the slowness with which ice and snow melt. When a thaw comes on when the heat is far above the degree of frost, though the ice is constantly surrounded by air warmer than itself, and constantly receiving heat from it, yet it will be hours, sometimes days, in dissolving. If nothing was necessary to produce fluidity but mere sensible heat, we might expect, that after it begins to melt a short time would be sufficient to melt the whole; but since it is so long in dissolving, and its heat is not increased above the freezing point, nor the water that runs from it above thirty-two or thirty-three degrees, this obvious heat, or, in more philosophical terms, this excess of caloric, to which it is exposed must be absorbed, and become latent. It is owing to this that ice can be preserved in ice-houses, and that large masses of ice and snow remain at the tops of mountains whose heat is considerably above the freezing point. If, on the other hand, we expose water to freeze, and put a thermometer into it, at first suppose it is twenty degrees warmer than the cold air, it will lose a great many degrees during the first five minutes, less the next, and so on; in half an hour (if the temperature of the air is below frost) it will have arrived at that temperature, and we should expect that in two or three minutes all of it would be frozen, if it depended only upon a diminution of sensible heat; but this is not the case, for we find at first a small part of it freezing and gradually increasing in the congelation. All this time the water continues at thirty-two degrees, which is perhaps one or two above the temperature of the air to which it is exposed. Now since it is known that if a colder body is applied to a warmer it will soon become of the same temperature, what prevents the water from becoming of the same temperature with the air to which it is exposed? It is doubtless owing to the caloric, which had been latent, emerging and becoming obvious or sensible as soon as any particle of the water freezes; and as soon as this is all exhausted, the mass becomes solid, and of the same temperature with the air. The quantity of caloric which thus emerges when a fluid body passes into a solid state might be estimated, if the temperature of the air would continue the same for a sufficient length of time. From all this it is plain that the combination of a certain portion (or dose, in the chemical language,) of caloric with ice turns it into water, and the removing or taking away of that portion of caloric converts it again into ice. Thus water is a compound of ice and of caloric; and indeed all fluids are combinations of the solid, and a certain dose of caloric. The quantity of caloric necessary to render ice at 32 fluid is 140°. There were disputes in Fahrenheit's time about the rarefaction of ice, whether it depended on the air contained in it during its fluidity. He imagined that if he extracted the air from water, he could produce an ice heavier than water. He extracted, therefore, the air from small glass globes filled with water, and after exposing them to an intense cold, they were a long time in freezing, though cooled greatly below the freezing point; but upon breaking them to examine them, the air rushed in, which, from the sudden shock, occasioned the water to freeze in a mo ment. He afterwards found that simple agitation had the same effect. If water is set at rest, it may be cooled several degrees, at seven, eight, nine, or ten, below the freezing point, without being congealed; but if touched with a bit of ice, or the end of a wire, or if the vessel is agitated, the congelation pervades it like a flash of lightning. This is confirmed by Mairan in his treatise of frost. He exposed small drinking-glasses full of water, which might be cooled below the freezing point, and if left undisturbed, might remain so, but upon being agitated they froze immediately; and if a thermometer was put to the water during its freezing, the moment it was frozen it rose up to the congealing point; therefore a quantity of latent caloric seems to have quitted the water: only part of the water was congealed into a spongy substance, which contained water in its pores; the water was eight or nine degrees below frost, and suddenly became warmer. Here we see a quantity of latent |