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

by Pierre Joseph Macquer

OBSERVATIONS.

In our Elements of the Theory, we explained how we imagine that Lime and other substances, which, according to the Table, have less affinity than Volatile Alkalis with Acids, are nevertheless capable of decompounding Sal Ammoniac, by uniting with its Acid, after expelling it from its basis, which is a Volatile Alkali. To recapitulate our opinion in two words: we conceive this to depend on the fixedness of these earthy and metallic additaments, which enables them to resist the force of fire, and on the volatility of the basis of Sal Ammoniac, which proves a great disadvantage to it when it comes to struggle, as it were, with those fixed additaments, aided by a considerable degree of heat. We shall only observe, that we are not singular in this opinion, nor indeed did we deliver it as a new one; that several modern Chymists concur with us therein, and particularly Mr. Baron, whom we have already mentioned more than once on the subject of Borax; and who, we think, was the first that ever took particular notice of it in print, viz. in his Memoirs on Borax, communicated to the Academy before the publication of our Elements. For the explanation of this phenomenon, therefore, we refer to those Memoirs, which are actually published, and to what we have already said on the subject in our treatise above-mentioned.

Another phenomenon, which is equally singular and curious, furnishes us with matter for several reflections, and gives us occasion to relate, in few words, the result of Mr. Duhamel's most sagacious experiments and speculations tending to discover the cause thereof. The point under consideration is the different forms and properties which the Volatile Alkali assumes, when separated from Sal Ammoniac by the means of a Fixed Alkali, and by the means of Lime. We know that the former is always in a concrete form, unless the mixture, from which it is distilled, be absolutely drenched with water; and that the latter, on the contrary, is always in a fluid form, and constantly liquid, whatever method be taken to distil it.

Some Chymists imagine, that the Volatile Salt of Sal Ammoniac appears in a concrete form, only because it still contains some Acid; whence they conclude that the reason why no concrete Volatile Salt can be obtained by the means of Lime is, because it absorbs all the Acid of the Sal Ammoniac; which is not the case, they say, with Fixed Alkalis. Others impute the constant fluidity of the Volatile Spirit of Sal Ammoniac, obtained with Lime, to the particles of fire which they suppose communicated thereto by that substance. Mr. Duhamel equally refutes both these opinions, by proving from experiments that Fixed Alkalis are capable of absorbing as much Acid as Lime can, and even more; and that, having been calcined as long, and with as violent a fire, as Lime, they must contain and communicate as many particles of fire; if indeed it be possible that the particles of fire should actually be lodged, and continue imprisoned, in calcined substances, as these gentlemen suppose. Yet this is contrary to experience; seeing the Volatile Salt distilled by the means of a Fixed Alkali, though ever so long and ever so violently calcined, is always in a concrete form, and doth not resemble the Volatile Spirit of Sal Ammoniac prepared with Lime.

In order to throw the necessary lights on this point, Mr. Duhamel had recourse to the only method that can be depended on in Natural Philosophy; namely, Experiments. He accordingly made several, of which these are the chief.

First, he distilled a Volatile Salt, by the means of well desiccated Salt of Tartar, and Salt of Soda; and, urging the fire with great violence towards the end of the operation, he thus obtained a quantity of Volatile Salt equal to, or even exceeding, that of the Sal Ammoniac he used: whence he justly concluded that, on this occasion, the Volatile Salt carried up, and volatilized some of the Fixed Salt.

Secondly, he found upon trial that the Volatile Spirit, obtained from Sal Ammoniac by the means of Lime, appears in the form of a liquor, only because it is mixed with some water which was contained in the Lime. Of this truth he had the following decisive proof: having attempted to prepare a Volatile Spirit of Sal Ammoniac with Lime, which had not been slaked, either in the air or by water, he could not obtain any Volatile Spirit: or, at least, the quantity was so small that it might be reckoned as nothing; and even that was wholly due to the moisture which Sal Ammoniac necessarily contains, together with that which Lime imbibes from the air, if ever so little exposed thereto.

From these two experiments Mr. Duhamel draws the following consequences: viz. that the Volatile Salt cannot be separated from the Sal Ammoniac and sublimed, without carrying along with it some of the additament which serves to extricate it; or, instead thereof, some other body with which it is capable of uniting: that Fixed Alkalis have the property of being thus carried up by the Volatile Alkali, and subliming with it: that the case is not the same with Lime, which therefore cannot, when alone, separate and sublime the Volatile Alkali of the Sal Ammoniac; but becomes capable thereof when it hath imbibed any moisture, which joins with the Volatile Salt, and rises therewith in distillation. And hence it must be concluded, that, seeing the Volatile Salt carries up with it some of the Fixed Alkali, by the means of which it is separated, it will be in a concrete form; what it carries up along with it being dry and solid: whereas, when it is distilled with Lime, it cannot but be liquid; seeing it must needs be dissolved by the moisture it gets from the Lime, without which it would not rise.

But to what must we attribute these effects produced by Lime, so different from those produced by Fixed Alkalis? Are they owing to its quality of Lime? or would it produce the same, if it were only a mere Absorbent Earth? Mr. Duhamel hath answered this question by a third sort of experiment. He tried to decompound Sal Ammoniac, and to separate its Volatile Alkali, by a pure Absorbent of Earth, without mixing any water with it, or calcining it.

For this purpose he made use of Chalk; and his experiment succeeded. By means of this additament he decompounded Sal Ammoniac, and by the experiment obtained the lights he wanted. The Volatile Alkali, being extricated by the dry but uncalcined Chalk, rose in a concrete form, as with Fixed Alkalis; and in like manner carried up with it some of the earthy additament. The same Chalk when calcined, and converted into Lime, produced the very effect of Lime on Sal Ammoniac. It is therefore from calcination alone that Absorbent Earths derive the property of retaining obstinately the Volatile Alkali, and preventing its sublimation by refusing to rise with it as Fixed Alkalis do.