Magnesia is not less remarkably distinguished from the calcarious earths, by joining it to the nitrous and vegetable acids, than to the vitriolic. Those earths, when combined with spirit of nitre, cannot be reduced to a crystalline form, and if they are dissolved in distilled vinegar, the mixture spontaneously dries up into a friable salt.
Having thus found magnesia to differ from the common alkaline earths, the object of my next inquiry was its peculiar degree of attraction for acids, or what was the place due to it in Mr. Geoffroy's table of elective attractions.
Three drams of magnesia in fine powder, an ounce of salt ammoniac, and six ounces of water were mixed together, and digested six days in a retort joined to a receiver.
During the whole time, the neck of the retort was pointed a little upwards, and the most watery part of the vapour, which was condensed there, fell back into its body. In the beginning of the experiment, a volatile salt was therefore collected in a dry form in the receiver, and afterwards dissolved into spirit.
When all was cool, I found in the retort a saline liquor, some undissolved magnesia, and some salt ammoniac crystallized. The saline liquor was separated from the other two, and then mixed with the alkaline spirit. A coagulum was immediately formed, and a magnesia precipitated from the mixture.
The magnesia which had remained in the retort, when well washed and dried, weighed two scruples and fifteen grains.
We learn by the latter part of this experiment, that the attraction of the volatile alkali for acids is stronger than that of magnesia, since it separated this powder from the acid to which it was joined. But it also appears, that a gentle heat is capable of overcoming this superiority of attraction, and of gradually elevating the alkali, while it leaves the less volatile acid with the magnesia.
Dissolve a dram of any calcarious substance in the acid of nitre or of common salt, taking care that the solution be rendered perfectly neutral, or that no superfluous acid be added. Mix with this solution a dram of magnesia in fine powder, and digest it in the heat of boiling water about twenty four hours; then dilute the mixture with double its quantity of water, and filtrate. The greatest part of the earth now left in the filtre is calcarious, and the liquor which passed thro', if mixed with a dissolved alkali, yields a white powder, the largest portion of which is a true magnesia.
From this experiment it appears, that an acid quits a calcarious earth to join itself to magnesia; but the exchange being performed slowly, some of the magnesia is still undissolved, and part of the calcarious earth remains yet joined to the acid.
When a small quantity of magnesia is thrown into a solution of the corrosive sublimate of mercury, it soon separates part of the mercury in the form of a dark red powder, and is itself dissolved.