When we have simplified our notions of the causes of change which happen to bodies under the distinction or division of chemistry, we must resolve them into two, namely, heat and attraction. Daily experience shows us that bodies may be more or less heated, and also that they adhere to each other. We are in truth unable to proceed farther in our abstrac tions. The causes of those well known effects have not yet been develoved by the manifestation of any more simple facts upon which they may depend. We can only observe the laws, according to which these powers have been found to act, and make our classification of the phenomena ; and as it is of some utility, in directing our future researches, to make conjectures by analogy, it may also be permitted to speculate upon the causes of these primary effects, provided it be done with caution, and without that bigotry, which even in systems of philo sophy has so frequently established the results of error.
Besides the effects of heat and attrac tion, we find that bodies are changed and modified by light, electricity, galvanism, and magnetism ; the three last of which are accompanied by attraction, or repul sion. But as these are much less gene rally applicable in operative chemistry than the powers first mentioned, and as it seems likely that future discoveries may lead to some intimate relation, or perhaps show the identity of the cause of heat, light, and the other affections of matter, which have here engaged our at tention, it is unnecessary to enlarge upon these in the present article.
The word attraction denotes the unex plained tendency which bodies have to move to each other. We observe it act ing at a distance in the fall of bodies on the surface of the earth, and in the mo tions of the heavenly bodies, as well as in such as are affected by electricity, gal vanism, or magnetism ; and in the cohe sion which gives solidity, or, more pro perly, rigidity, to bodies, as well as in those effects wherein the parts of differ ent bodies unite to form new compounds, we deduce its effects from motions or ac tions, which cannot be separately distin guished. And these differences, though they cannot be shown to arise from one and the same power, or from energies originally dissimilar, require, at least for the purposes of language, to be treated apart from each other. Chemistry seems to have little to do with the perceptible attractions : it is principally confined to the state of bodies, as it relates to the cohesion and the combination of their parts.
Heat, or rather temperature, is a well known modification of bodies, by which they produce a peculiar sensation, dis tinguished by the same word. Its laws have been very successfully investigated by our contemporaries ; for which see CA LORIC, HEAT, and COMBUSTION. The ope rative chemist considers it as the means of converting solid bodies into dense fluids, and dense fluids into elastic fluids, called gas or vapour, while compound bo dies may have their parts separated from each other by this treatment.
When bodies of different kinds are brought into contact, they produce very little of the change called chemical, while they continue in the solid state. Mechanical trituration will forward their mutual action, by multiplying the sur faces of contact ; but still the masses con tinue too large to be moved amongst each other by the peculiar attractions they may be capable of exerting. It has been considered as an axiom in chemis try, that bodies do not act on each other, unless one or both be in the fluid state ; and though this is not strictly and uni versally true, yet it is requisite for almost every operation of chemistry, that this condition, either of dense or of elastic fluidity, should obtain. The facility with which the parts of fluids move amongst each other is, no doubt, the principal cause of this increased effect.
The practical part of chemistry may be therefore said to consist almost entirely in separating or changing the order of the parts of bodies by heat, or of placing bodies in such situations with regard to each other, as that, with the assistance of heat, if needful to produce fluidity ; changes or separations of the same kind may take place among their parts. The actions of electricity, galvanism, and light, will probably be soon combined among the leading resources of chemis try.
No change could take place by this or any other treatment, if the attractions of the parts of bodies to each other were all perfectly the same. It is manifest from the facts, that the attractions between some bodies is stronger than between others, and from this remarkable variety in the habitudes of bodies, the attractions of chemistry have been called elective attractions.
A distinction has been made between those processes in which water is present, and those in which the requisite fluidity is produced by strong heat. The first me. thod is called the humid way, and the other the dry way.