9. Nitric acid was known to him by the name of dissolving water. He prepared it by putting into an alembic one pound of sulphate of iron of Cyprus, half a pound of saltpetre, and a quarter of a pound of alum of Jameni: this mixture was distilled till every thing liquid was driven over. He mentions the red fumes which make their appearance in the alembic during the process.[122] This process, though not an economical one, would certainly yield nitric acid; and it is remarkable, because it is here that we find the first hint of the knowledge of chemists of this most important acid, without which many chemical processes of the utmost importance could not be performed at all.
10. This acid, thus prepared, he made use of to dissolve silver: the solution was concentrated till the nitrate of silver was obtained by him in a crystallized state. This process is thus described by him: “Dissolve silver calcined in solutive water (nitric acid), as before; which being done, coct it in a phial with a long neck, the orifice of which must be left unstopped, for one day only, until a third part of the water be consumed. This being effected, set it with its vessel in a cold place, and then it is converted into small fusible stones, like crystal.”[123]
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.
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.
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.
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]
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]
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.”
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]
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.