"Take of nitrate of potassa in powder, and of sublimed sulphur, equal weights. Mix them well together, and throw the mixture in small quantities at a time into a red-hot crucible. The deflagration being finished, let the salt cool, and preserve it in a well-stopped glass vessel."

In this process the sulphur is oxidized, and converted partly into sulphuric acid, and partly into sulphurous acid, by uniting with the oxygen afforded by the decomposition of the nitric acid of the nitrate, which is effected by the degree of heat employed. During the deflagration, however, a part of the acid is volatilized in the form of nitrous oxide, and consequently the oxygen evolved is not sufficient to acidify all the sulphur, and the unaltered portion remains united with a portion of potassa. The sulphuric and sulphurous acids combine with the remainder of the potassa; and thence the product is a mingled mass, consisting of sulphate and bisul-phate of potassa and sulphuret of potassa. It is the preparation which was originally known under the name of sal polychrest.

Qualities. - This salt has a sensibly acid taste, and reddens infusion of litmus. It is almost wholly dissolved in eight parts of water, at 60°; and by exposure to the air it is altogether converted into sulphate of potassa.

Medical properties and uses. - The same as those of sulphate of potassa; and consequently it is scarcely ever used. It passes through the kidneys undecomposed.