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

by Pierre Joseph Macquer

In the experiments made to neutralize Crystal of Tartar, Fixed Alkaline Salts alone were formerly used. Messrs. Duhamel and Grosse were the first who discovered that Absorbent Earths might be substituted for Alkalis, and would produce nearly the same effects on Crystal of Tartar. The experiments made by these two Academicians in conjunction are circumstantially related in two curious Memoirs on this subject, given in by them jointly, and printed with those of the Academy for 1732 and 1733. From these Memoirs we took the process here given, and shall also borrow from thence most of the remarks we are now going to make.

Stone-lime holds, as it were, the middle place between mere Absorbent Earths and Fixed Alkalis. Now, seeing Crystal of Tartar may be converted into a Neutral Salt by either of these two substances, it follows, that lime ought to produce the same effect upon it. Accordingly Messrs. Duhamel and Grosse found it to be so upon trial: having formed, with Lac calcis and Crystal of Tartar, a Neutral Salt perfectly like that which results from the union of that saline matter with Chalk. Cremor calcis, or that salino-terrene pellicle which forms on lime-water, produced the same effect: but, what is most singular is, that lime-water itself, though it be clear and limpid, and consequently doth not seem to contain any earthy particles, produced nevertheless a great effervescence with Crystal of Tartar, and neutralized it as perfectly as Cremor calcis, or water ever so much impregnated with Chalk. This arises from hence, that a great quantity of the salino-terrene matter, which forms the Cremor calcis is dissolved in the lime-water.

Though lime-water neutralizes Crystal of Tartar as perfectly as Chalk does, and though the Crystals of soluble Tartar, or neutralized Tartar, thereby produced, be like those which have Chalk for their basis, yet Messrs. Duhamel and Grosse observed some differences, worthy of notice, between the phenomena accompanying the production of these two Neutral Salts, which resemble each other so much that they seem but one and the same species of Salt. The principal difference consists in this, that the water containing the Tartar neutralized by Chalk is very limpid, and leaves but a very small quantity of earth on the filter; whereas the lime-water, with which Tartar hath been neutralized, leaves on the filter a considerable quantity of earth.

This must appear the more surprising, that the water replete with Chalk was, before its union with the Crystal of Tartar, turbid and opaque; whereas the Lime water was at first clear and limpid. Messrs. Duhamel and Grosse suspect this to arise from hence, that the effervescence excited, while the Crystal of Tartar dissolves the matter contained in lime-water, is greater than that which is produced by its union with Chalk suspended in water.

"If we consider," say they, "that in a great effervescence a considerable quantity of the acid Spirit is evaporated, we shall easily perceive, that, the more of that Spirit escapes, the more of the earth of the Tartar will be precipitated. Now, as the effervescence with lime-water is more considerable, and as there is less alkaline earth to check, as it were, and restrain the Acid, than in the experiment with Chalk, a greater quantity of the acid Spirit may escape; which being entirely lost will cause more earth to precipitate in this case than in the other, where the Acid is all at once attracted by a great deal of alkaline earth: and accordingly this was the reason that our Tartar dissolved by Chalk deposited, in crystallizing, a grey earth, which was scarce perceivable in the experiment made with Lime-water.

"Yet perhaps," say they, "an Acid, which we suspect to be contained in lime, may have partly occasioned the precipitation of this earth." The existence of this Acid, which these gentlemen at that time only suspected, hath been since demonstrated by several experiments, and particularly by those which Mr. Malouin hath published. This Acid is the Vitriolic, which, in combination with some of the earth of the lime, forms a sort of Selenitic Salt; which adds greatly to the probability of Messrs. Duhamel and Grosse's last conjecture. I shall now explain how I conceive the Vitriolic Acid in lime may occasion the copious precipitate which falls in lime-water, when Crystal of Tartar is neutralized by it.

The quantity of Vitriolic Acid contained in lime is very inconsiderable; so that to convert it into a Neutral Salt requires its intimate union with a very small quantity of the earthy and absorbent parts. Hence it comes to pass, that, when water is poured upon quick-lime, in order to make the Lime-water, it in some sort divides the lime into two parts. All the particles of Absorbent Earth, which had not contracted an union with the Acid, are at first barely suspended in the liquor, the transparency of which they destroy, giving it an opaque white colour; and this is what makes the Lac calcis: but they soon separate from it, and fall to the bottom, in the form of a precipitate; because they are not soluble in water. By this precipitation the liquor becomes limpid, and remains impregnated only with such of the earthy parts as are united with the Vitriolic Acid, in the form of a kind of Neutral Salt, and have by that union acquired solubility. But the Vitriolic Acid finding many more Absorbent parts in the lime than were necessary to neutralize it, in a manner over-dosed itself with earthy parts, and thereby exceeded the bounds of a perfect Neutrality.

On the other hand, it hath been shewn, that Crystal of Tartar is an imperfect Neutral Salt. Now these two Salts, which are neither of them perfectly Neutral, differ from a perfectly Neutral Salt by properties directly opposite to each other; seeing the Selenitic matter in Lime exceeds in its absorbent or alkaline quality, and Crystal of Tartar exceeds, on the contrary, in acidity.

What must be the consequence, therefore, of mixing these two saline matters together? The same as when an Acid is mixed with a Fixed Alkali; that is, the Salt which exceeds in acidity will combine with the super-abundant alkaline earth of the Selenitic Salt; so that these two saline matters will both become perfectly Neutral Salts. Yet these two Neutral Salts have not the same degree of solubility in water. The neutralized Crystal of Tartar dissolves very readily in water, and is for that reason called Soluble Tartar: the Selenitic Salt, on the contrary, is hardly dissolvable in it at all. Now it is a rule that, when two Salts of this nature meet together, the most soluble always remains united with the water, exclusive of the other, which is forced to precipitate. This I imagine to be what happens in the present case; and the precipitate which we see fall, in the lime-water employed to neutralize Crystal of Tartar, seems to me to be no other than the Selenitic Salt of the Lime; which, being less soluble than the neutralized Tartar, gives place to it, and separates from the liquor.

Indeed we cannot, in my opinion, account for the precipitate under consideration, any other way, than by supposing it to be a portion either of the earth of the Crystal of Tartar, or a portion of the Lime. Now, either of these earths is dissolvable by Acids; whereas the precipitate in question, according to the observations of Messrs. Duhamel and Grosse, is not so: and this ought to be the case, if the precipitate be nothing but the selenitic Salt of the Lime, which being a Neutralized Salt, partly constituted by the most powerful of all the Acids, must be unalterable by any Acid whatever.