Their affinity with acids is greater than that of absorbent earths; and hence it comes to pass, that if an Alkali be presented to a combination of an Acid with an absorbent earth, the earth will be separated from the Acid by the Alkali, and a new union between the Acid and the Alkali will take place. This is both an instance and a proof of our fifth proposition concerning Affinities.
If a pure Alkali be presented to a pure Acid, they rush together with violence, and produce the same phenomena as were observed in the union of an absorbent earth with an Acid; but in a greater and more remarkable degree.
Fixed Alkalis may in general be divided into two sorts: one of these hath all the above-recited properties; but the other possesses some that are peculiar to itself. We shall consider this latter sort more particularly under the head of Sea-Salt.
SECTION III.
Of Neutral Salts.
The Acid and the Alkali thus uniting mutually rob each other of their characteristic properties; so that the compound resulting from their union produces no change in the blue colours of vegetables, and has a taste which is neither sour nor acrid, but saltish. A saline combination of this kind is for that reason named Sal Salsum, Sal Medium, of a Neutral Salt. Such combinations are also called by the plain general name of Salts.
It must be observed that, in order to make these Salts perfectly Neutral, it is necessary that neither of the two saline principles of which they are compounded be predominant over the other; for in that case they will have the properties of the prevailing principle. The reason is this: neither of these saline substances can unite with the other but in a limited proportion, beyond which there can be no further coalition between them. The action by which this perfect union is accomplished is termed Saturation; and the instant when such proportions of the two saline substances are mixed together, that the one is incorporated with as much of the other as it can possibly take up, is called the Point of Saturation. All this is equally applicable to the combination of an Acid with an absorbent earth.
The combination is known to be perfect, that is, the Point of Saturation is known to be obtained, when, after repeated affusions of an Acid in small quantities to an Alkali, or an absorbent earth, we find those phenomena cease, which in such cases constantly attend the conflict of union, as we said above, namely, ebullition, hissing, &c. and we may be assured the Saturation is complete when the new compound hath neither an acid nor an acrid taste, nor in the least changes the blue colours of vegetables.
Neutral salts have not so great an affinity with water as either Acids or Alkalis have; because they are more compounded: for we observed before, that the affinities of the most compounded bodies are generally weaker than those of the most simple. In consequence hereof few Neutral Salts, when dried, attract the moisture of the air; and those that do, attract it more slowly, and in less quantity, than either Acids or Alkalis do.
All Neutral Salts are soluble in water; but more or less readily, and in a greater or smaller quantity, according to the nature of their component principles.