Experiments upon magnesia alba, Quicklime, and some other Alcaline Substances

by Joseph Black

I also rendered the volatile alkali caustic, in order to examine what change it suffered in the operation, and obtained an exceedingly volatile and acrid spirit, which neither effervesced with acids, nor altered in the least the transparency of lime-water; and, altho' very strong, was lighter than water, and floated upon it like spirit of wine.

I next inquired into the truth of the fifth proposition, in the following manner.

Two drams of epsom-salt were dissolved in a small quantity of water, and thrown into two ounces of the caustic-ley; the mixture instantly became thick, like a decoction of starch or barley, by the magnesia, which was precipitated. I then added spirit of vitriol by degrees, until the mixture became perfectly clear, or the whole of the magnesia was again dissolved; which happened without any effervescence or emission of air.

Half an ounce of chalk was dissolved in spirit of salt, the quantity of which was so adjusted, that the mixture was not acid in the least degree; and the solution was thrown into twelve ounces of the caustic ley; which quantity I found, by experiment, to be sufficient for precipitating almost the whole of the chalk. I now filtrated this turbid liquor, and laid the powder remaining in the paper upon a chalk-stone, in order to draw as much of the water from it as possible, and thereby reduce it to the form of a more dense and heavy powder, that it might subside the more perfectly in the following part of the experiment. I then mixed it with about twenty ounces of pure water in a flask, and, after allowing the powder to subside, poured off the water, which had all the qualities of lime-water. And I successively converted eight waters more into lime-water, seven of these in the same quantity, and with the same management, as the first. The eighth was likeways in the same quantity; but I allowed it to remain with the chalk, and shook it frequently, for two days. This, after being filtrated, formed a cream or crust upon its surface when exposed to the air; changed the colour of the juice of violets into green; separated an orange-coloured powder from a solution of corrosive sublimate; became turbid upon the addition of an alkali; was entirely sweetened by magnesia; and appeared so strong to the taste, that I could not have distinguished it from ordinary lime-water. And when I threw some salt ammoniac into the lime which remained, the vapour of the volatile alkali immediately arose from the mixture.

In this experiment therefore the air is first driven out of the chalk by an acid, and then, in order to separate this acid from it, we add an alkali which has been previously deprived of its air; by which means, the chalk itself is also obtained free of air, and in an acrid form, or in the form of slaked lime.

We have also several processes for obtaining the volatile alkali in a caustic form, which seem to be only so many methods of obtaining it in its pure state, and free of fixed air. The first of these is the separation of the alkali from an acid, merely by heat; an instance of which we have from Mr. Margraaf.[9] He prepared from urine an ammoniacal salt, the acid of which is the basis of the phosphorus, and is of such a peculiar nature, that it endures a red heat without being dissipated. Sixteen ounces of the neutral salt were subjected by him to distillation. The acid remained in the retort, and he found in the receiver eight ounces of an alkaline spirit, which, he tells us, was extremely volatile, very much resembling the spirit of salt ammoniac distilled with quick-lime; and no crystals were formed in it, when exposed to the cold air.

A caustic volatile alkali may also be obtained, by mixing salt ammoniac with half its weight of a caustic fixed alkali, or of magnesia which has been previously deprived of its air by fire; and then submitting these mixtures to distillation: Or merely by adding any ordinary volatile alkali to a proper quantity of a caustic ley; for in this case the air passes from the volatile to the fixed alkali, by a superior attraction for the last, and, by a gentle heat, the compound yields a spirit similar to that prepared from salt ammoniac and quick-lime.

It is therefore probable, that, had we also a method of separating the fixed alkali from an acid, without, at the same time, saturating it with air, we should then obtain it in a caustic form; but I am not acquainted with an instance of this separation in chemistry. There are two indeed which, at first sight, appear to be of this kind; these are the separation of the fixed alkali from the nitrous acid by means of inflamed charcoal, in the process for making nitrum fixatum, and of the same alkali, from vegetable acids merely by heat; but, upon examining the product of each process, we find the alkali either fully or nearly saturated with air. In the first, either the charcoal or the acid, or both together, are almost wholly converted into air; a part of which is probably joined to the alkali. In the second, the acid is not properly separated, but rather destroyed by the fire: a considerable portion of it is converted into an inflammable substance and we learn from Dr. Hales, that the bodies of this class contain a large quantity of fixed air.

When we consider that the attraction of alkalis for fixed air is weaker than that of the calcarious earths, and reflect upon the effects of heat in chemistry, we are led to imagine, that alkalis might be entirely deprived of their air, or rendered perfectly caustic, by a fire somewhat weaker than that which is sufficient to produce the same change upon lime; but this opinion does not seem agreeable to experience.