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.
The alkalis do, however, acquire some degree of causticity in a strong fire, as appears from their being more easily united with spirit of wine after having been kept in fusion for some time. For that fluid, which cannot be tinctured by a mild salt of tartar, will soon take a very deep colour from a few drops of a strong caustic ley. The circumstances which hinder us from rendering these salts perfectly caustic by heat, are their propensity to dissipation in the utmost violence of the fire, their extreme acrimony, and the imperfection of our common vessels. For before the heat becomes very intense, the alkalis either evaporate, or dissolve a part of the crucibles in which they are contained, and often escape thro' their pores; which happens, especially as soon as they have already acquired some degree of additional acrimony, by the loss of part of their air.
The fusion also, which they so readily undergo, is well known by Chemists, as a strong obstacle to the separation of the volatile from the fixed parts of a compound by fire; accordingly, in several processes, we are directed to add to the fusible compound some porous substance which is incapable of fusion, and will retain the whole in a spongy form, thereby to facilitate the dissipation of the volatile parts.
In order to know whether an alkali would lose a part of its air, and acquire a degree of causticity, when exposed, with this precaution, to the action of a strong fire, I mixed an ounce and a half of salt of tartar with three ounces of black-lead, a substance of any the most unchangeable by chemical operations. This mixture I exposed, for several hours, in a covered crucible, to a fire somewhat stronger than what is necessary to keep salt of tartar in fusion. When allowed to cool, I found it still in the form of a loose powder; and taking out one half, I diluted it with water, and by filtration obtained a ley, which, when poured into a solution of white marble in _aqua fortis_, precipitated the marble under the form of a weak quick-lime: for the turbid mixture gave a green colour to the juice of violets, and threw up a crust like that of lime-water; and the precipitated powder collected and mixed with salt ammoniac immediately yielded the scent of the volatile alkali.