SION.
All substances in nature, as far as we are acquainted with them, occur in one or other of the three following states ; namely, the state of solids, of liquids, or of elastic fluids or vapours. It has been ascertained, that in avast number of cases, the same substance is capable of exist ing successively in each of these states.
All solid bodies, a very small number ex cepted, may be converted into liquids by heating them sufficiently ; and, on the other hand, every liquid, except spirit of wine, is convertible into a solid body, by exposing it to a sufficient degree of cold. All liquid boks may, by heating them, be converted into elastic fluids, and a great many solids are capable of under going the same change ; and lastly, the number of elastic fluids, which by cold are condensible into liquids or solids, is by no means inconsiderable. These facts have led philosophers to form this general conclusion, "that all bodies, if placed in a temperature sufficiently low, would as sume a solid form ; that all solids be come liquids when sufficiently heated; and that all liquids, when exposed to a certain temperature, assume the form of elastic fluids." The state of bodies then depends upon the temperature in which they are placed ; in the lowest tempera tures they are all solid ; in higher tempe ratures they are converted into liquids ; and in the highest of all they become elastic fluids. The particular tempera tures at which bodies undergo those changes are exceedingly various, but they are always constant for the, same bodies. Thus we see that heat produces changes on the state of bodies, convert ing them all, first into liquids, and then into elastic fluids.
When solid bodies are converted by heat into liquids, the change in some cases takes place at once. There is no interval between solidity and liquidity ; but in other cases a very gradual change may be perceived : the solid becomes first soft, and it passes through all the degrees of softness, till at last it be comes perfectly fluid. The conversion of ice into water is an instance of the first change ; for in that substance there is no intervening state between solidity and fluidity. The melting of glass, of wax, and of tallow, exhibits instances of the second kind of change ; for these bodies pass through every degree of softness before they terminate in per fect fluidity. In general, those solid bo dies which crystallize or assume regular prismatic figures, have no interval be tween solidity and fluidity ; while those that do not usually assume such shapes have the property of appearing succes sively in all the intermediate states.
Caloric not only increases the bulk of bodies, and changes their state from solids to liquids, and from liquids to elastic fluids, but its action decomposes a great number of bodies altogether, either into their elements, or it causes these elements to combine in a different manner. Thus, when ammonia is heated to redness, it is resolved into azotic and hydrogen gases. Alcohol, by the same heat, is converted into carbureted hydro gen and water.
This decomposition is in many cases owing to the difference between the vo latility of the ingredients of a compound. Thus, when weak spirits, or a combina tion of alcohol and water, are heated, the alcohol separates, because it is more vo latile than the water. In general, the compounds, which are but little or not at all affected by heat, are those bodies which have been formed by combustion. Thus water is not decomposed by any heat which can be applied to it ; neither are sulphuric, phosphoric, or carbonic acids. Almost all the combinations into which oxygen enters, without having oc casioned combustion, are decomposable by heat. This is the case with nitric acid, hyperoxymuriatic acid, and many of the metallic oxides.
All bodies that contain combustibles as component parts are decomposed by heat. Perhaps the metallic alloys are exceptions to this rule ; at least it is not in our power to apply a temperature high enough to produce their decomposition, except in a few cases.
When two combustible ingredients and likewise oxygen occur togetberin bodies. they are always very easily decomposed by heat. This is the case with the greater number of animal and vegetable substances.
Having examined the nature, and some few of the properties and effects of calo ric, as far as the subject has been hitherto investigated, it now only remains for us to mention the different methods by which caloric may be evolved or made sensible, or the different sources from which it may be obtained. These sour ces may be reduced to five : it radiates constantly from the sun ; it is evolved during combustion ; and it is extricated in many cases by percussion, friction, and mixture. The sources of heat, then, are, the SUN, COMBUSTION, PERCUSSION, FRIC