Though these ingenious experiments evidently furnish us with great lights, for discovering the cause of the solidity or fluidity of the Volatile Alkali, when separated from Sal Ammoniac by different additaments, as they fully determine several preliminary questions immediately relating thereto; yet they still leave us, in some measure, at a loss with regard to the chief point. For we do not yet know why Fixed Alkalis and Absorbent Earths, which, in all Chymical trials, shew that they have certainly as much fixity as Lime, are carried up by the Volatile Alkali, while Lime resists, instead of rising with it as those other substances do, obstinately retains it, and even fixes it in some measure, so that it is impossible for it to sublime. This question, in my opinion, depends on the theory of Lime; nor can we hope to resolve it in its full extent, till we get a further insight into the nature of that singular substance than we have at present.
On this subject, however, Mr. Duhamel hath offered some conjectures, founded on the known properties of Lime, and supported by experiments. "Lime," says he, "is an earth freed by calcination from almost all its humidity, almost all its Acid, and all the fat it contained; whether that fat came from some animal parts, as is the case of those stones which consist of shells; or whether it were a bituminous fat, as may happen to be the case with some others: this substance is withal acrid and fiery; it is very greedy of moisture, and imbibes it when exposed thereto. It absorbs Acids, and retains them strongly; and, lastly, it unites with fat matters, and therewith makes a kind of soap."
All these properties are verified by experiments; and therefore Mr. Duhamel thinks he hath a right to say, that Lime acts not only on the Acid of Sal Ammoniac, but also on the fatty matter which always accompanies Volatile Alkalis, and is essential to their nature; and therefore it decompounds them. Of this Mr. Duhamel gives the following convincing proof, founded on experiment. He took some Volatile Spirit distilled with Lime, and abstracted it several times from a fresh parcel of Quick-Lime. The quantity of the Spirit diminished sensibly every time; and the Lime was at last so replete with fat, that the Vitriolic Acid, when poured thereon, became very sulphureous; and moreover, when calcined in a crucible, it emitted a very perceptible smell of burnt grease.
Indeed Fixed Alkalis are also capable of absorbing and retaining fat matters; but not near so strongly as Lime: because these Salts are never entirely freed from that which they contain originally; whereas Lime seems much poorer, and absolutely void of any oily matter.
On these principles Mr. Duhamel resolved to try if he could not obtain a Volatile Alkali in a concrete form, by distilling the Volatile Spirit from Lime, brought nearly to the condition of a Fixed Alkali, by imbibing a portion of fat matter. With this view he distilled a great quantity of Volatile Spirit from a little Lime, and actually obtained a small portion of Volatile Salt; because the great quantity of Volatile Spirit had, in some measure, saturated the Lime with fat matter.
Mr. Duhamel tried also to bring Lime back to the condition of a pure Absorbent Earth, to decalcine it, if I may use the term; in order to try whether he could not by this means make it produce the same effect as Chalk. For this purpose he lixiviated some Lime four months successively, pouring every day fresh water on it, and removing that of the preceding day, together with the crystalline crust which always formed on it; and after leaving this Lime two years in the shade, he applied it to Sal Ammoniac. It produced a moderate quantity of Volatile Salt, which was very transparent, and seemed to be crystallized in cubes. Thus we see Lime rendered very like Chalk. Yet it was pretty acrid on the tongue, and the Volatile Salt, obtained by its means, was more disposed to run into a liquid than that separated by Chalk: which shews that this Lime still retained some part of its former character, and that its transformation was not complete.
To conclude what relates to the Volatile Alkali of Sal Ammoniac, it only remains that we say a word or two of that portion of the earthy or saline additament, which, though fixed in its nature, sublimes nevertheless with the Volatile Alkali, and gives it a concrete form.
Mr. Duhamel, who, in every subject that he handles, omits nothing worthy of attention, made several other experiments, with a view to discover whether or no the Salt of Tartar, and the Chalk, carried up by the Volatile Alkali, be truly volatilized; and whether or no there be such a strict union contracted, between the Urinous Salt and these fixed substances, that the whole results in what is called a Concrete Volatile Salt; or if those fixed substances be united but superficially with the Urinous Salt, which only carries them up along with itself in sublimation, as Sal Ammoniac carries up several very fixed metallic matters.
The result of the experiments made by Mr. Duhamel for this purpose is, that the fixed substances carried up by the Volatile Alkali of the Sal Ammoniac are actually volatilized; that they make, as it were, one whole with it; and are so closely combined therewith, that almost all the most efficacious means of separating fixed from volatile matters are unsuccessful with regard thereto. Nothing, for instance, is fitter to separate a volatile substance from a fixed one, than to mix the compound with a great quantity of water, and to distil the whole, with such a degree of heat as shall be exactly sufficient to elevate the volatile part. In this manner Mr. Duhamel treated Volatile Alkalis replete with Fixed Salt, and with Chalk: but though he applied no more than the gentlest degree of heat; nay, exposed his mixture to the air only, fearing lest he should make the heat too strong if he used fire; yet the fixed part, which the Volatile Salt had carried up with it, continued still united therewith; so that the whole passed over in distillation, or was dissipated by evaporation, without leaving any thing fixed at the bottom of the vessel.