ALUM.--This is what is termed a double salt, and is composed of sulphate of alumina and sulphate of potash. The process of manufacturing it in this country is by subjecting clay slate containing iron pyrites to a calcination, when the sulphur with the iron is oxidized, becoming sulphuric acid, which, combining with the alumina of the clay, and also with the iron, becomes sulphate of alumina and iron; to this is added a salt of potash, which, combining with the sulphate of alumina, forms the double salt alum. Soda or ammonia may be substituted for potash with similar results; the alum is crystallized from the solution. That the ancients were acquainted with this double salt has been disputed, but we think there can be no doubt of its existence and use at a very early period. A very pure alum is produced in volcanic districts by the action of sulphurous acid and oxygen on felspathic rocks, and used by the ancients for different purposes. Pliny mentions _Alumine_, which he describes as white, and used for whitening wool, also for dyeing wool of bright colors. Occasionally he confounds this salt with a mixture of sulphate of alumina and iron, which, in all probability, was alum containing iron, the process of separation not being perfect; and he mentions that this kind of alumen blackens on the application of nut-galls, showing that iron was in it. Pliny says of alumen, that it is "understood to be a sort of brine which exudes from the earth; of this, too, there are several kinds. In Cyprus there is a white alumen, and another kind of a darker color; the uses of these are very dissimilar, the white liquid alumen being employed for dyeing a whole bright color, and the darker, on the other hand, for giving wool a tawny or sombre tint." This is very characteristic of a pure aluminous mordant, and of one containing iron. He also mentions that this dark alumen was used for purifying gold. He must be referring here to its quality of giving gold a rich color. The liquid of this iron alumen, if put upon light-colored gold, and heated over a fire, gives it a very rich tint; a process practiced still for the same purpose. So far, however, as the application to dyeing is concerned, it is unnecessary to prove that the ancients used our double salt alum. Probably the alumen referred to by Pliny, as exuding from the earth, was sulphate of alumina, without potash or soda, a salt not easily crystallized, but as effective, in many cases more effective, in the operations of dyeing, as alum, which is attested by the preference given to this salt over alum for many purposes at the present day. Pliny says that alumen was a product of Spain, Egypt, Armenia, Macedonia, Pontus, Africa, and the Islands of Sardinia, Melos, Lipara, and Strangyle, and that the most esteemed is that of Egypt. And Herodotus mentions that King Amasis of Egypt sent the people of Delphi a thousand talents of this substance, as his contribution toward the rebuilding of their temple.