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we have hithefto acquired of the method by which combinations are formed. I can readily conceive that the point at which water is saturated with a salt may change according to the temperature, and that cold water may absorb more of the gaseous acid; the increase or diminution of the matter of heat in the solvent changing the respective disposition, the density, and perhaps the figure of the molecules, it is not astonishing that the attractive force should be modified by these changes; that they should result in a contact more or less perfect, and that the power of affinity should be able by this means to protect from (the influence of) the law of gravitation a greater quantity of matter, in the one case, than in the other; but we have nothing of this kind in the hypothesis under consideration; the circumstances are the same; the point of saturation cannot change, because it is the effect of a cause which does not change.
“ To make this clear, let us ask what is saturation ? Every chemist will say that he understands by it the condition in which a compound is, when· neither of its constituents can receive or retain in combination a greater quantity of the other. Such is the rational and necessary acceptation of the word saturation, otherwise it becomes void of sense; then to suppose, preserving this acceptation, that a substance may be saturated with different quantities of the same substance, is really to affirm two contraries.”
Now, when a new quantity of any of their principles is added to one of these perfectly neutral salts, there is nothing to prevent an attraction between them, and even in a degree capable of producing solution, combination, or affinity ; but it must be remarked that this affinity is not that of an acid to a base, or a base to an acid, but of the neutral compound, with the portion that was added; whence it follows; 1st, that it has no effect on the previous composition which remains in its integrity, as if the neutral salt were super
compounded with a foreign body; 2nd, that the point of saturation is not changed; and 3rd, that the power which unites this portion which is added to the neutral salt, may be much weaker than that which unites the same substance to the same, at the point of saturation, without causing a contradiction.”
Page 566. “I shall terminate this section by a short résumé of the principal characters, which are capable of forming a methodic division of affinities, and which may have been lost sight of in the course of the preceding discussions.
“ Ist. Two bodies of the same nature, whether simple or compound, may unite and form a third, as homogeneous as either of the two before the union. This is called affinity of aggregation.
“2nd. Two bodies of a different nature, simple or compound, may unite without undergoing any change in their first composition, if they are compound. This is called affinity of composition.
“ Two out of three bodies may shew a preference and combine, leaving the third at liberty; these bodies may either be simple or compound, provided their composition does not change, and they are found in a condition favourable to contact. This is still affinity of composition; whether the three bodies have been put separately into the mixture, or two been previously united, whilst the superior affinity of the third has destroyed their union, by what is called precipitation.
“ Three or more bodies, exposed to contact, may unite in such a manner as to form only one homogeneous mass. This is nevertheless affinity of composition, although two only unite at first, and a third is added to the first two, and so on successively.
“ 3rd. Two bodies incapable of combining, become so when one or the other has either been decomposed or supercomposed.
In this case, the affinity of composition, which produces the union, takes the name of disposing affinity.
“ 4th. Two or more compounds being placed in circumstances, suitable for bringing into play the respective affinities of their component parts; either there is a change and new products formed without our being able to determine which is the most powerful affinity under which they act, or the first composition remains, contrary to the order indicated by the superior affinity of the principle of one of the component parts to the affinity of the other. In these two cases we say that they are not the relations of affinity of one body to another, but affinities of concourse, otherwise called double affinities; in a word, the sum of all the united affinities which are needed to explain these phenomena.
“ Two bodies being put in contact, the compound which results is supercompounded, or united with an excess of one of the principles. This tendency to supercomposition, is sometimes so strong, that when the least affinity of a third body interferes with it, the proportions of the first compound are changed, and the neutral state destroyed. This tendency may cause the production of crystals, with excess of base, in an acid liquor. To distinguish this force, we shall call it the affinity of a compound for an excess of one of its constituents, or for shortness, affinity of excess, which will be enough to recall the idea when it has been well grasped. However paradoxical some of these propositions may appear, I have no fear of their being called in question after the proofs I have given; and if they are well founded, it will readily be granted that they ought to form one of the most important elements in the calculation of affinities."
Here now we have what Morveau has given, as the most certain of the laws of affinities known among chemists at his time. He adds, however, that they certainly scarcely deserve the title. The following are additions :
*“Ist. There is no chemical union, if one of the bodies be not sufficiently fluid to allow the molecules to obey the law of affinity, which carries them from (mere) proximity to (actual) contact.
“2nd. Affinity takes place only among the smallest integral molecules of a body.
“ 3rd. From the affinity of one substance with another, we cannot know the affinity of a compound; we cannot know the affinity of a compound of one of these substances, with one or the other in excess.
“ 4th. The affinity of composition has no efficacy, unless it can overcome the affinity of cohesion.
“ 5th. Two or more bodies, which unite by the affinity of composition, form a substance which has new properties distinct from those which belong to each of the bodies before combination.
“6th. To give effect to affinities, a particular temperature is necessary, which renders the action either slow or rapid, invalid or efficacious.”
These feeble and incipient attempts at laws may surely surprise us, written as they are almost within the memory of some of the living
There was at this time an attempt to measure the force of affinities as the only method of obtaining results, and there was not yet seen the absolute necessity of having bodies kept uniform by a constant and absolutely similar composition, much less to bring this absoluteness of composition under the forms of natural law, and we might add also, logical necessity.
It should be noticed that the remarks about saturation shew that compound proportion was not in the least degree understood in the present sense. Its possibility is even denied.
The aim was to find the strength of affinity. When it is
* Remarks in italics on the various laws, from p. 567 onwards.
said that we cannot find the affinity of a compound from knowing that of its parts, it refers to affinity dynamically; no hint is given as to the knowledge of this amount quantitatively. Proportion generally may be said to be excluded.
Although these opinions give a pretty fair idea of the state of the chemical mind at the several dates, we must return to an earlier date again to follow up another direction.