The consideration of what happens to the parts of bodies, in consequence of their elective attractions, constitutes the most difficult part of the science, whe ther the mind be employed in developing the facts, or in deducing the general theory which may be indicated from them, It is, therefore, necessary'to con sider them with some attention, and in a regular manner.
The adhesion of parts, considered to be of the same kind, is called aggrega tion. Thus a number of pieces of glass melted together form an aggregate : And the smallest parts into which an aggre gate can be imagined to be divided, so as uot to change its nature, are called in tegrant parts; so that the integrant parts of glass are themselves glass. But when the body is known to be•made up of parts of different kinds or nature, and it is considered with regard to these, the body is called a compoond, or combina tion, and the parts are called component parts or principles. In this manner glass is a compound of the earth called silex. and a salt called alkali, combined together at a strong heat : and we may imagine, that if there were any means by which glass could be reduced, first to its integrant parts, and the division could be carried farther, the parts would then be no longer integrant and glass, but would become divided into compo nent parts, namely, earth, and alkali. Bodies are also considered in a wide manner by the name of mixtures, when small aggregates of different kinds are united, as in a variety of minerals, where the parts are frequently distinguishable by the senses: and in the arts we have sand and lime made into mortar by mix ture, or sand, clay, and other earths, made into pottery, and hardened by a moderate fire; but these by a stronger heat may be made to combine into glass, andare then no longer mixtures, but com pounds.
The early chemists were led into a sup position, that the bodies they were un able to analyze were simple, and they distinguished them by the name of ele ments. It is probable that the great va riety of bodies around us are formed by combination, out of a few simple princi ples, or perhaps out of one single ele ment, variously combined as to figure and position of parts; but iris useless and un profitable to speculate on probabilities, which experiment can never verify. Mo
dern chemists, very properly, consider those bodies as simple, which have not yet been decomposed; but this is merely with relation to the present state of our knowledge, and for the sake of ar rangement and induction. They do not lose sight of the necessity of instituting experiments for their farther analysis; and the great discoveries which have done honour to our own times, are a proof of their diligence and sagacity.
We do not know of any means of as certaining, by experiment, whether com pound bodies do enter as principles into other bodies still more compounded ; or whether in bodies of three or more prin ciples, all the simple particles do dis pose themselves without any dependance on the order of time, according to which they may have been put together. It is probable that the former is the case, so that we may hereafter be enabled to de signate primary principles, or bodies not yet decomposed ; secondary principles, or bodies of two primary principles, which nevertheless can enter into com bination, or be disengaged without sepa ration from each other ; ternary princi ples, &c. In this manner sulphur, by combining with oxygen and water, will form sulphuric acid, and this acid may be combined with a metal, so as to form a salt capable of giving out its acid again by heat. Our systematic books are written according to the supposition of seconda ry and more complicated principles ; but the facts do not indisputably prove their existence.
When two bodies in the solid state which are disposed to combine are brought into contact, the combination will begin at the place where they touch ; and if the compound be of such a nature, as that its freezing point (see Caxonic) is lower than the common temperature of the bo dies, it will he fluid, and the combination may proceed to the other parts of each, till the whole shall have united. Thus snow and salt will form a fluid brine, if the temperature be higher than 6° be low the commencement of Fahrenheit's scale.