11. He was in the habit also of dissolving sal ammoniac in this nitric acid, and employing the solution, which was the aqua regia of the old chemists, to dissolve gold.[124] He assures us that this aqua regia would dissolve likewise sulphur and silver. The latter assertion is erroneous. But sulphur is easily converted into sulphuric acid by the action of aqua regia, and of course it disappears or dissolves. [124] Ibid., chap. 23. 12. Corrosive sublimate is likewise described by Geber in a very intelligible manner. His method of preparing it was as follows: “Take of mercury one pound, of dried sulphate of iron two pounds, of alum calcined one pound, of common salt half a pound, and of saltpetre a quarter of a pound: incorporate altogether by trituration and sublime; gather the white, dense, and ponderous portions which shall be found about the sides of the vessel. If in the first sublimation you find it turbid or unclean (which may happen by reason of your own negligence), sublime a second time with the same fuses.”[125] Still more minute directions are given in other parts of the work: we have even some imperfect account of the properties of corrosive sublimate. [125] Invention of Verity, chap. 8. 13. Corrosive sublimate is not the only preparation of mercury mentioned by Geber. He informs us that when mercury is combined with sulphur it assumes a red colour, and becomes cinnabar.[126] He describes the affinities of mercury for the different metals. It adheres easily to three metals; namely, lead, tin, and gold; to silver with more difficulty. To copper with still more difficulty than to silver; but to iron it unites in nowise unless by artifice.[127] This is a tolerably accurate account of the matter. He says, that mercury is the heaviest body in nature except gold, which is the only metal that will sink in it.[128] Now this was true, applied to all the substances known when Geber lived. [126] Sum of Perfection, book i. part iii. chap. 4. [127] Ibid., chap. 6. [128] Ibid. He gives an account of the method of forming the peroxide of mercury by heat; that variety of it formerly distinguished by the name of _red precipitati per se_. “Mercury,” he says, “is also coagulated by long and constant retention in fire, in a glass vessel with a very long neck and round belly; the orifice of the neck being kept open, that the humidity may vanish thereby.”[129] He gives another process for preparing this oxide, possible, perhaps, though certainly requiring very cautious regulation of the fire. “Take,” says he, “of mercury one pound, of vitriol (sulphate of iron) rubified two pounds, and of saltpetre one pound. Mortify the mercury with these, and then sublime it from rock alum and saltpetre in equal weights.”[130] [129] Sum of Perfection, book i. part iv. chap. 16. [130] Invention of Verity, chap. 10.