It will be shewn in its proper place, that almost every vegetable matter, as well as Tartar, leaves a fixed alkali in its ashes: yet Tartar has these peculiar properties; first, it assumes an alkaline character even when burnt or calcined in close vessels, whereas other substances acquire it only by being burnt in the open air; secondly, the alkali of Tartar is stronger and more saline than almost any that is obtained from other matters.
This alkali, when thoroughly calcined, powerfully attracts the moisture of the air, and melts into an unctuous alkaline liquor, improperly called Oil of Tartar per deliquium. This is the alkali generally used in making the Terra Foliata, mentioned under the head of Vinegar; for which reason this combination is called Terra Foliata Tartari; a name suitable enough. But the same cannot be said of the other name, Regenerated Tartar, which is also given it. It is true, that on this occasion an oily acid is restored to the earth of the Tartar, analagous to that of which the fire had deprived it: but the compound thence resulting is a neutral salt which very readily dissolves in water; whereas Tartar is manifestly acid, and not soluble, or at least hardly soluble, in water.
Crystals of Tartar combined with alkali of Tartar produce a great effervescence while they are mixing, as all acids usually do; and if the combination be brought exactly up to the point of saturation, a perfectly neutral salt is formed, which shoots into crystals, and easily dissolves in water; and this hath procured it the name of Soluble Tartar. It is also called the Vegetable Salt, as being obtained from vegetables only; and again, Tartarised Tartar, because it consists of the acid and the alkali of Tartar combined together.
Crystals of Tartar combined with alkalis procured from the ashes of maritime plants, such as Soda, which alkalis resemble the basis of sea-salt, form likewise a neutral salt, which crystallizes well, and dissolves easily in water. This salt is another sort of soluble Tartar. It is called Saignette's Salt, from the inventor's name.
Both the Vegetable Salt and Saignette's Salt are gently purgative soaps, and much used in Medicine.
Tartar likewise dissolves the absorbent earths, as lime, chalk, &c. and with them forms neutral salts which are soluble in water[3]. It even attacks metallic bodies, and when combined with them becomes soluble. A soluble Tartar for medical use is prepared with Crystals of Tartar and Iron: the metallic salt thereby produced hath the name of Chalybeated Soluble Tartar. This salt attracts the moisture of the air, and is one of those which do not crystallize.
Crystallized Tartar acts also upon several other metallic substances: for instance, it dissolves the Regulus, Liver, and Glass of Antimony, and thence acquires an emetic quality: it is then called Stibiated or Emetic Tartar. It likewise dissolves Lead, and therewith forms a salt which, in the figure of its crystals, resembles Tartarised Tartar.
It is very extraordinary that Tartar, which of itself is not soluble in water, should be soluble therein when become a neutral salt by uniting either with alkalis or with absorbent earths, or even with metals. With respect to alkalis, indeed, it may be urged, that, having themselves a great affinity with water, they communicate to Tartar some of that facility with which they naturally unite therewith: but the same cannot be alledged concerning absorbent earths, and metallic substances, which water dissolves not at all, or at least with great difficulty, and in small quantity. This effect, therefore, must be attributed wholly to some change in the disposition of its parts which is to us unknown.
All the Soluble Tartars are easily decompounded by exposing them to a certain degree of heat. In distillation they yield the same principles which are obtained from Tartar; and what remains fixed in the fire, after they are thoroughly burnt, is a compound of the alkali which Tartar naturally produces, and of the alkaline or metallic substance with which it was converted into a neutral salt.
As Crystal of Tartar is the weakest of all acids, on account of the oily and earthy matters with which it is combined, Soluble Tartars are decompounded by all the acids; by any of which crystal of Tartar may be separated from the substance that serves it for a basis and renders it a neutral salt.