3. We now arrive at the third of the effects of heat, the change which it produces in the state of bodies, convert ing, according to circumstances, a solid into a liquid, or a liquid into an aeriform fluid. The effect of caloric, which we last described, consists in introducing a portion of heat between the particles of bodies, by which they are, to a certain extent, separated from each other, so as to experi ence an increase of bulk, without having their cohesion materially impaired. lf, however, the addition of heat be continued beyond a certain limit, the particles are removed still farther from each other, until at length they are so far separated, as to lose their cohesive power, and to become easily movable among themselves in all directions. By this change of form, the substance in question undergoes a complete alteration in its physical properties, and not un frequently in its chemical action upon other bodies. The effect is produced by destroying the balance between the expansive power of heat, and the force of attraction : the former tending to remove the particles of bodies to a distance from each other, the latter to retain them in close contact. But although the attractive force is conceived to be very much weakened in bodies when they assume the fluid state, it is not entirely destroyed, but is still exerted with considerable energy.
It is also to be remarked, that after a solid has received an addition of heat, so as to convert it into a fluid, a still' farther quantity of heat may be given to it, which has the simple effect of expansion. lf, however, we continue to increase the quantity of heat after the expansion has ad vanced to a certain length, the body again assumes a new form, and becomes a gas ; and it now, as before, after this third change, by adding still more heat, suffers merely an expansion in its volume. But the expansion in these three cases, as has been already observed, differs considerably in its degree, being small in the first instance, greater in the second and much more so in the last. This peculiar quali ty of assuming the different states of solidity, liquidity, and elastic fluidity, may be considered as a property common to all kinds of matter ; for it would appear, that if we had the power of producing, at pleasure, temperatures suffi ciently high and sufficiently low, every substance that usu ally appears under the solid form might be vaporized, and every substance that usually appears as a vapour might be rendered solid. When we speak, therefore, of any body be ing naturally in the solid, or naturally in the liquid state, we mean no more than that, under the ordinary temperature of the atmosphere, or under the circumstances in which it commonly presents itself to our notice, it is solid or fluid.
There is, indeed, another obstacle to altering the state of bodies, besides the difficulty of procuring very high or very low temperatures, that by a great addition or subtrac tion of heat, their chemical constitution is affected, and they are either decomposed, or are disposed to enter into new combinations. Organised substances in general, both of animal and vegetable origin, when subjected to high tem peratures, are converted into their ultimate elements ; and, on the contrary, in some cases, a great reduction of tem perature causes the constituent parts of certain bodies to separate from each other, before they are brought into the solid form. These, however, are to be regarded rather as incidental circumstances, interfering with particular operations, than as any fundamental exceptions to the ge neral law that has been laid down. It would appear from
the experiments of Sir James Hall, that many of these ap parent exceptions may be brought under the general rule, merely by employing strong pressure or other mechanical means. Thus it has been supposed impossible to melt the carbonate of lime, because at an elevated temperature the carbonic acid escaped in the form of gas; but when the sub stance was subjected to strong compression, so as to prevent this escape of gas, the fusion was easily accomplished.
The suddenness of the change which occurs when the body receives so much heat as to convert it into the liquid state, slicws that some other circumstance takes place be sides the mere addition of caloric. And the same remark applies to vaporization; for here again no indication of the change of state can be observed, until the actual change occurs in its full extent. This point we shall soon consider more at length ; but, in the mean time, before we quit the subject of the change of bodies from the solid to the liquid form, we must offer a few observations upon some pheno mena that occur when we reverse the operation, and con vert them from the liquid to the solid state. All bodies that undergo this change, without having their chemical constitution materially altered, have a fixed point at which they are said to freeze, congeal, or become solid ; and ex cept under certain circumstances, this point is invariably the same. This is so much the case with respect to water, that the freezing point of this fluid has been employed as one of the fixed degrees which regulate the graduation of the whole scale.
It is, however, possible, by certain management, to cool water several degrees below its -freezing point, without rendering it solid; according to the experiments of Mr Dalton, as low as 5°, or 27° below the usual point of congelation. In order to succeed in giving water this unusual degree of cold, without converting it into ice, it is necessary that it be kept in a state of the most complete rest : for the least agitation either prevents it from falling lower than 32°, or if it be brought down below this point, it instantly begins to freeze, and the fluid part rises to 32°. Black found that by carefully boiling the water, so as to expel from it all the air which is generally dissolved in it, it was less easy to reduce the temperature below the ordi nary degree ; and Blagden discovered, that when water is thus artificially cooled, no circumstance had more effect in producing its sudden congelation, than introducing into it a small particle of ice. Hence, when the freezing of water has once commenced, it proceeds with so much rapidity in these experiments ; and it may perhaps also in some mea sure, account for the effect which water confined in close vessels has in bursting the vessel when it freezes. In these cases the fluid may probably descend some degrees below the usual freezing point, before the congelation commences; a slight agitation, or other incidental circumstance, then causes the freezing to commence, and the first spi culm of ice that are formed tend to convert the remaining part of the water into the solid state so rapidly, as to pro duce a greater expansion, than if the particles had arrang ed themselves more quietly in the crystalline form.