On the contrary, the integrant parts of bodies do not absolutely differ from each other; nor do they differ, as to the nature and principles, from the body into whose mass they enter. By the integrant parts of a body are to be understood the smallest molecules or particles into which this body can be reduced without decomposition. We may conceive that a neutral salt, for instance, common salt, may be divided into molecules still smaller and smaller, without any separation of the acid and alkali which constitute the salt; so that these molecules, however small, shall always be common salt, and possessed of all its essential properties. If we should now suppose that these molecules are arrived at their utmost degree of smallness, so that each of them shall be composed of one atom of acid and of another atom of alkali, and that they cannot be further divided without a separation of the acid and alkali, then these last molecules are those which Mr Macquer in his Chemical Lectures calls primary integrant molecules.
In the same manner that we conceive that a body may be divided into its primary integrant molecules without any change of its nature, or other alteration than a diminution of its bulk; so we may also easily perceive, that if these primary integrant molecules, which are all homogeneous and of the same nature, and which are supposed separated from each other, should be brought to unite and combine together, no new body, that is, a body of different nature, will result from this union, but only a more considerable mass of the same body: that is to say, for instance, that if the primary integrant molecules were common salt, their reunion would still form common salt, only in a mass so much larger, as there is a greater number of these molecules united together. But it is the union of these homogeneous parts, of these primitive integrant molecules, which modern chemists have called aggregation; and they have called aggregates bodies considered as resulting from their primary integrant parts, in opposition to the names mixt and compounds, which they have given to bodies considered as resulting from the union of their constituent parts, which are substances heterogeneous and of different natures.
The name of integrant parts, which has been given to those whose union forms aggregates, agrees will with them; because, in fact, this union is a kind of addition or integration (if such a word may be used) of a certain number of parts of the same kind, whence results a sum, or a whole.
It is very essential to observe on the subject of aggregation, that we should have a very false idea of it, and entirely opposite to chemical phenomena, if we understood by this word nothing but a simple juxtaposition of the integrant parts of bodies: for besides that, there must be a real adhesion and intimate union of these same parts with each other, so that they cannot be separated but by some force superior to that by which they are united. A heap of sand, for example, if we consider the grains of sand as its integrant parts, cannot be regarded as an aggregate, because these grains are only juxtaposited, and have no real adhesion together; so that the resistance which they oppose to their separation can only proceed from their gravity, and is not the effect of adhesion, or tendency to each other.
In the second place, it is necessary to observe on the subject of aggregation, that the force of adhesion of the integrant parts of different bodies varies much, according to the nature of these bodies, some of them adhering very strongly, and others very weakly; and that those which adhere weakly are generally the easiest to be dissolved, considering that the solution of a body, or its combination with another body of a different nature, cannot be effected but so far as the integrant parts of these bodies are separated, or their aggregation is broken; which is partly done by the operations of art, but chiefly by the action of menstruums.
All these considerations on the aggregation of bodies, although very simple in themselves, are of the greatest importance in the theory and practice of chemical operations. Becker and Stahl were the first who explained these ideas, which have been since illustrated and extended by the best modern chemists.
Philosophers and chemists have long ago discovered, that almost all natural bodies are capable of being reduced to a greater or less number of other bodies, less compounded, similar to each other, and almost the same, of whatever nature the body was from which they were separated. This important observation has induced a belief, that the innumerable productions of nature were only the results of the combination of a few more simple substances, the different proportions and arrangements of which formed the diversity of all compound bodies. These last bodies retain the name of compounds; and the more simple substances, from the union of which these compounds result, are called principles: but as the number and nature of he principles of bodies. Accordingly, in each school of philosophy different opinions prevailed concerning these principles, some of them admitting one only, and others more: some asserting that water is the principle of all things; some contending for earth, and others for fire. We do not intend to discuss all these opinions; but shall only observe, that almost all the ancient philosophers have been mistaken from want of a sufficient number of chemical experiments and observations.
That earth, water, and fire enter into the composition of bodies as principles, may be considered now as demonstrated Becher and Stahl. The experiments of many philosophers and chemists, particularly Boyle and Hales, have shown that air also enters into the composition of many bodies as a principle, and even in very large quantities. Thus if we add this fourth principle to the three others abovementioned, we shall find to our surprise, that we now admit as principles of all compounds the four elements, fire, air, water, earth, which Aristotle taught were such, long before the knowledge of chemistry was sufficient to establish this truth.
In whatever manner bodies are decomposed, we always obtain these substances. They are the utmost limits of chemical analysis. As we cannot by any means decompose these further, we consider them as simple substances, (although perhaps they may not be so) and are therefore called primary principles or elements.[...]
When we decompose most bodies, we cannot nearly reduce them to their elements or primary principles by a first analysis, especially when they are much compounded. We obtain from them substances more simple only, but still compounded of other principles, for a reduction to which another analysis is requisite. As these substances, although compounded of a certain number of principles, do themselves the office of principles in the composition of bodies less simple than in themselves, they have been called principiate principles. The principate principles have so much better title to this name, as they subsist in their state after they have been separated from a body, characterized by peculiar properties, unalterable but by a new analysis, and capable of reproducing by their union a compound entirely like that from which they were originally separated. Most chemical agents, as acids and alkalis, are of this kind.
In the analysis of very compound bodies, principiate principles of different degrees of simplicity, or rather such as are capable of being reduced to others more and more simple, may be thus obtained successively, by first, second and third analyses: hence several kinds of principiate principles of different degrees of simplicity, and which consequently are, by a true gradation, principles of one another, have been distinguished. Modern chemists distinguish them by names which mark their order of composition. Thus substances which cannot be further decomposed, and which are considered as simple, are called primary principles. Secondary principles are those which result immediately from the union of primary principles. Principles of the third order are those which are composed of secondary principles; and so on.
We may also with propriety distinguish the principles of bodies by the names of proximate principles and remote principles, by applying the former name to the principiate principles obtained directly from a body by a first analysis, and by applying the latter name to the principles obtained by a further decomposition of these proximate principles.
These distinctions will be rendered more intelligible by an example; for the subject of which let us choose a neutral salt, such as nitre. We have demonstrated that this salt is a compound of the acid called nitrous, and of the fixed vegetable alkali, combined and saturated together. By a first analysis then of nitre we shall obtain this acid and this alkali, which are therefore the proximate principles of nitre. But neither nitrous acid nor fixed alkali are simple substances. By a further analysis of each of these, they may be decomposed into water, earth, fire or inflammable principle. This acid and this alkali must then be considered as principiate principles. But as the substances obtained from them are unalterable, and cannot be further decomposed, they must be considered as primary principles; the acid and the alkali of the nitre are then immediately composed of primary principles, and they themselves are consequently principiate principles, or secondary principles, or principles of the second order. In this example the water, earth and fire are the remote principles of the nitre.
Although these several denominations and distinctions of principles more or less simple be just and useful for the illustration of many important points in the theory of chemistry, this science is not however sufficiently advanced to enable us to determine the number and kind of principiate principles of different orders, and especially of the higher orders. We know but a few, which, we have strong reasons to believe, belong to the second order; such, for example, are saline substances, and the more simple acids and alkalis; but we are not entirely certain of this, because we have not been able to produce any of these substances from the union of the primary principles, in such a manner, that no doubt can be admitted.