14. Geber was acquainted with several of the compounds of metals with sulphur. He remarks that sulphur when fused with metals increases their weight.[131] Copper combined with sulphur becomes yellow, and mercury red.[132] He knew the method of dissolving sulphur in caustic potash, and again precipitating it by the addition of an acid. His process is as follows: “Grind clear and gummose sulphur to a most subtile powder, which boil in a lixivium made of ashes of _heartsease_ and quicklime, gathering from off the surface its oleaginous combustibility, until it be discerned to be clear. This being done, stir the whole with a stick, and then warily take off that which passeth out with the lixivium, leaving the more gross parts in the bottom. Permit that extract to cool a little, and upon it pour a fourth part of its own quantity of distilled vinegar, and then will the whole suddenly be congealed as milk. Remove as much of the clear lixivium as you can; but dry the residue with a gentle fire and keep it.”[133] [131] Sum of Perfection, book i. part iii. chap. 4. [132] Ibid. [133] Invention of Verity, chap. 6. 15. It would appear from various passages in Geber’s works that he was acquainted with arsenic in the metallic state. He frequently mentions its combustibility, and considers it as the _compeer_ of sulphur. And in his book on _Furnaces_, chapter 25 (or 28 in some copies), he expressly mentions _metallic arsenic_ (_arsenicum metallinum_), in a preparation not very intelligible, but which he considered of great importance. The white oxide of arsenic or arsenious acid, was obviously well known to him. He gives more than one process for obtaining it by sublimation.[134] He observes in his Sum of Perfection, book i. part iv. chap. 2, which treats of sublimation, “Arsenic, which before its sublimation was evil and prone to adustion, after its sublimation, suffers not itself to be inflamed; but only resides without inflammation.” [134] Invention of Verity, chap. 7. Geber states the fact, that when arsenic is heated with copper that metal becomes white.[135] He gives also a process by which the white arseniate of iron is obviously made. “Grind one pound of iron filings with half a pound of sublimed arsenic (arsenious acid). Imbibe the mixture with the water of saltpetre, and salt-alkali, repeating this imbibation thrice. Then make it flow with a violent fire, and you will have your iron white. Repeat this labour till it flow sufficiently with peculiar dealbation.”[136] [135] Sum of Perfection, book ii. part. ii. chap. 11. [136] Invention of Verity, chap. 14. 16. He mentions oxide of copper under the name of _æs ustum_, the red oxide of iron under the name of _crocus_ of iron. He mentions also litharge and red lead.[137] But as all these substances were known to the Greeks and Romans, it is needless to enter into any particular details. [137] Ibid., chap. 4 and 12.