Elements of the Theory and Practice of Chymistry, 5th ed.

by Pierre Joseph Macquer

It is easy to infer, from what has been said of Salts in general, that some of them must be more, some less, fixed or volatile than others, and some more, some less, disposed to unite with water, with earth, or with particular sorts of earth, according to the nature or the proportion of their principles.

Before we proceed further, it is proper just to mention the principle reasons, which induce us to think that every Saline Substance is actually a combination of earth and water, as we supposed at our entering on this subject. The first is, the conformity Salts have with earth and water, or the properties they possess in common with both. Of these properties we shall treat fully, as occasion offers to consider them, in examining the several sorts of Salts. The second is, that all Salts may be actually resolved into earth and water by sundry processes; particularly by repeated dissolution in water, evaporation, desiccation, and calcination. Indeed the Chymists have not yet been able to procure a Saline Substance, by combining earth and water together. This favours a suspicion, that, besides these two, there is some other principle in the composition of salts, which escapes our researches, because we cannot preserve it when we decompose them; but it is sufficient to our purpose, that water and earth are demonstrably amongst the real principles of Saline Substances, and that no experiment hath ever shewn us any other.

SECTION I.

Of Acids.

Of all Saline Substances, the simplest is that called an Acid, on account of its taste; which is like that of verjuice, sorrel, vinegar, and other sour things, which, for the same reason, are also called Acids. By this peculiar taste are Acids chiefly known. They have moreover the property of turning all the blue and violet colours of vegetables red, which distinguishes them from all other salts.

The form, under which Acids most commonly appear, is that of a transparent liquor; though solidity is rather their natural state. This is owing to their affinity with water; which is so great, that, when they contain but just as much of it as is necessary to constitute them Salts, and consequently have a solid form, they rapidly unite therewith the moment they come into contact with it: and as the air is always loaded with moisture and aqueous vapours, its contact alone is sufficient to liquify them; because they unite with its humidity, imbibe it greedily, and by that means become fluid. We therefore say, they attract the moisture of the air. This change of a salt from a solid to a fluid state, by the sole contact of the air, is also called Deliquium; so that when a salt changes in this manner from a solid into a fluid form, it is said to run per deliquium. Acids being the simplest species of Saline bodies, their affinities with different substances are stronger than those of any other sort of salt with the same substances; which is agreeable to our fourth proposition concerning Affinities.

Acids in general have a great affinity with earths: that with which they most readily unite is the unvitrifiable earth to which we gave the name of absorbent earth. They seem, not to act at all upon vitrifiable earths, such as sand; nor yet upon some other kinds of earths, at least while they are in their natural state. Yet the nature of these earths may be in some measure changed, by making them red-hot in the fire, and then quenching them suddenly in cold water: for, by repeating this often, they are brought nearer to the nature of absorbent earths, and rendered capable of uniting with Acids.

When an acid liquor is mixed with an absorbent earth, for instance with chalk, these two substances instantly rush into union, with so much impetuosity, especially if the acid liquor be as much dephlegmated, or contain as little water, as may be, that a great ebullition is immediately produced, attended with considerable hissing, heat, and vapours, which rise the very instant of their conjunction.

From the combination of an acid with an absorbent earth there arises a new compound, which some Chymists have called Sal Salsum; because the Acid by uniting with the earth loses its sour taste, and acquires another not unlike that of the common sea-salt used in our kitchens; yet varying according to the different sorts of Acids and earths combined together. The Acid at the same time loses its property of turning vegetable blues and violet colours red.

If we inquire what is become of its propensity to unite with water, we shall find that the earth, which of itself is not soluble in water, hath, by its union with the Acid, acquired a facility of dissolving therein; so that our Sal Salsum is soluble in water. But, on the other hand, the Acid hath, by its union with the earth, lost part of the affinity it had with water; so that if a Sal Salsum be dried, and freed of all superfluous humidity, it will remain, in that dry solid form, instead of attracting the moisture of the air, and running per deliquium, as the Acid would do if it were pure and unmixed with earth. However, this general rule admits of some exceptions; and we shall have occasion in another place to take notice of certain combinations of Acids with earths, which still continue to attract the moisture of the air, though not so strongly as a pure Acid.