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

by Pierre Joseph Macquer

But it is proper to observe, that a Volatile Alkali decompounds such neutral salts as have for their basis either an absorbent earth or a metallic substance, without the aid of fire; whereas absorbent earths or metallic substances will not decompound an Ammoniacal Salt, unless they be assisted by a certain degree of heat.

Now, as all these matters are extremely fixed, at least in comparison with a Volatile Alkali, they have the advantage of being able to resist the force of fire, and so of acting in conjunction therewith; and fire greatly promotes the natural action of substances upon one another: whereas the Volatile Alkali in the Ammoniacal Salt, being unable to abide the force of fire, is compelled to desert its acid; and that so much the more quickly, as its affinity therewith is considerably weakened by the presence of an earthy or metallic substance, both of which have a great affinity with acids.

These considerations oblige us to conclude, that Volatile Alkalis have a somewhat greater affinity, than absorbent earths and metallic substances, with acids.

Ammoniacal Salts projected upon nitre in fusion make it detonate; and the Nitrous Sal Ammoniac detonates by itself, without the addition of any inflammable matter. This singular effect evidently demonstrates the existence of an oily matter in Volatile Alkalis; for it is certain that nitre will never deflagrate without the concurrence, and even the immediate contact, of some combustible matter.

This oily substance is often found combined with Volatile Alkalis in such a large proportion as to disguise it, in some measure, and render it exceedingly foul. The salt may be freed from its superfluous oil by repeated sublimations; and particularly by subliming it from absorbent earths, which readily drink up oils. This is called the Rectification of a Volatile Alkali. The salt, which before was of a yellowish or dirty colour, by being thus rectified becomes very white, and acquires an odour more pungent and less fetid than it had at first, that is, when obtained by one single distillation from a putrid substance.

It is proper to observe, that the rectification of a Volatile Alkali must not be carried too far, or repeated too often; for by that means it may be entirely decomposed at length; and particularly if an absorbent earth, and especially chalk, be employed for that purpose, the salt may be converted into an oil, an earth, and water.

Volatile Alkalis act upon several metallic substances, and particularly on copper; of which they make a most beautiful blue solution. On this property depends a pretty singular effect, which happens sometimes when we attempt, by means of a Volatile Alkali, to separate copper from any acid with which it is combined. Instead of seeing the liquor grow turbid, and the metal fall, both which generally happen when any Alkali whatever is mixed with a metallic solution, we are surprised to observe the solution of copper, upon adding a Volatile Alkali, retain its limpidity, and let fall no precipitate; or at least, if the liquor does grow turbid, it remains so but for a moment, and instantly recovers its transparency.

This is occasioned by adding such a quantity of Volatile Alkali as is more than sufficient fully to saturate the acid of the solution, and considerable enough to dissolve all the copper as fast as it is separated from the acid. On this occasion the liquor acquires a deeper blue than it had before; which arises from the property which Volatile Alkalis have of giving this metal, when combined with them, a fuller blue than any other solvent can: hence we have a touchstone to discover copper wherever it is; for let the quantity of this metal combined with other metals be ever so small, a Volatile Alkali never fails to discover it, by making it appear of a blue colour.

Though a Volatile Alkali be constantly the result of putrefaction, yet it must not therefore be imagined that none can be produced by any other means; on the contrary, most of those substances which contain the ingredients necessary to form it, yield no inconsiderable quantity thereof in distillation. Tartar, for example, which by being burnt in an open fire is converted, as was shewn, into a Fixed Alkali, yields a Volatile Alkali when it is decomposed in close vessels; that is, when it is distilled; because, in this latter case, the oily part is not dissipated or burnt, as it is by calcination in a naked fire, but has time to unite with some of the earth and acid of the mixt, in such a manner as to form a true Volatile Alkali.