4. Sal ammoniac was known to Geber, and seems to have been quite common in his time. There is no evidence that it was known to the Greeks or Romans, as neither Dioscorides nor Pliny make any allusion to it. The word in old books is sometimes _sal armoniac_, sometimes _sal ammoniac_. It is supposed to have been brought originally from the neighbourhood of the temple of Jupiter Ammon: but had this been the case, and had it occurred native, it could scarcely have been unknown to the Romans, under whose dominions that part of Africa fell. In the writings of the alchymists, sal ammoniac is mentioned under the following whimsical names: Anima sensibilis, Aqua duorum fratrum ex sorore, Aquila, Lapis aquilinis, Cancer, Lapis angeli conjungentis, Sal lapidum, Sal alocoph. Geber not only knew sal ammoniac, but he was aware of its volatility; and gives various processes for subliming it, and uses it frequently to promote the sublimation of other bodies, as of oxides of iron and copper. He gives also a method of procuring it from urine, a liquid which, when allowed to run into putrefaction, is known to yield it in abundance. Sal ammoniac was much used by Geber, in his various processes to bring the inferior metals to a state of greater perfection. By adding it or common salt to aqua fortis, he was enabled to dissolve gold, which certainly could not be accomplished in the time of Dioscorides or Pliny. The description, indeed, of Geber’s process for dissolving gold is left on purpose in a defective state; but an attentive reader will find no great difficulty in supplying the defects, and thus understanding the whole of the process. 5. Alum, precisely the same as the alum of the moderns, was familiarly known to Geber, and employed by him in his processes. The manufacture of this salt, therefore, had been discovered between the time when Pliny composed his Natural History and the eighth century, when Geber wrote; unless we admit that the mode of making it had been known to the Tyrian dyers, but that they had kept the secret so well, that no suspicion of its existence was entertained by the Greeks and Romans. That they employed _alumina_ as a mordant in some of their dyes, is evident; but there is no proof whatever that _alum_, in the modern sense of the word, was known to them.