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BOOK X.

consist of half a libra of brick dust, a quarter of a libra of salt, an uncia of salt-petre, half an uncia of sal-ammoniac, and half an uncia of rock salt. The bricksor tiles from which the dust is made must be composed of fatty clays, free fromsand, grit, and small stones, and must be moderately burnt and very old.

Another cement is made of a bes of brick dust, a third of rock salt, anuncia of saltpetre, and half an uncia of refined salt. Another cement is madeof a bes of brick dust, a quarter of refined salt, one and a half unciae ofsaltpetre, an uncia of sal-ammoniac, and half an uncia of rock salt. Anotherhas one libra of brick dust, and half a libra of rock salt, to which some add asixth of a libra and a sicilicus of vitriol. Another is made of half a libra ofbrick dust, a third of a libra of rock salt, an uncia and a half of vitriol, andone uncia of saltpetre. Another consists of a bes of brick dust, a third ofrefined salt, a sixth of white vitriol 19 , half an uncia of verdigris, and likewisehalf an uncia of saltpetre. Another is made of one and a third librae of brickdust, a bes of rock salt, a sixth of a libra and half an uncia of sal-ammoniac,a sixth and half an uncia of vitriol, and a sixth of saltpetre. Another containsa libra of brick dust, a third of refined salt, and one and a half unciae of vitriol.

The process as here described falls into five operations : a, granulation of the bullionor preparation of leaves ; b, heating alternate layers of cement and bullion in pots; c,washing the gold to free it of cement; d, melting the gold with borax or soda; e,treatment of the cement by way of melting with lead and cupellation to recover the silver.Investigation by Boussingalt (Ann. De Chimie, 1833, p. 253-6), D’Elhuyar ( Bergbaukunde ,Leipzig, 1790, Vol. 11, p. 200), and Percy (Metallurgy of Silver and Gold, p. 395), of theaction of common salt upon silver under cementation conditions, fairly well demonstratedthe reactions involved in the use of this species of cement. Certain factors are essentialbesides salt : a, the admission of air, which is possible through the porous pots used ; b, thepresence of some moisture to furnish hydrogen ; c, the addition of alumina or silica. Thefirst would be provided by Agricola in the use of new pots, the second possibly by use of woodfuel in a closed furnace, the third by the inclusion of brickdust. The alumina or silica athigh temperatures decomposes the salt, setting free hydrochloric acid and probably also freechlorine. The result of the addition of vitriol in Agricola’s ingredients is not discussed bythose investigators, but inasmuch as vitriol decomposes into sulphuric acid under hightemperatures, this acid would react upon the salt to free hydrochloric acid, and thus assistto overcome deficiencies in the other factors. It is possible also that sulphuric acid undersuch conditions would react directly upon the silver to form silver sulphates, which wouldbe absorbed into the cement. As nitric acid is formed by vitriol and saltpetre at high tem-peratures, the use of these two substances as a cementing compound would produce nitricacid, which would at once attack the silver to form silver nitrate, which would be absorbed intothe melted cement. In this case the brickdust probably acted merely as a vehicle for theabsorption, and to lower the melting point of the mass and prevent fusion of the metal.While nitric acid will only part gold and silver when the latter is in great excess, yet whenapplied as fumes under cementation conditions it appears to react upon a minor ratio ofsilver. While the reactions of the two above species of compounds can be accounted for in ageneral way, the problem furnished by Agricola’s statements is by no means simple, f° ronly two of his compounds are simply salt cements, the others being salt and nitre mixtures.An inspection of these compounds produces at once a sense of confusion. Salt is present inevery compound, saltpetre in all but two, vitriol in all but three. Lewis (Traite Singulier deMetallique, Paris, 1743,11, pp. 48-60), in discussing these processes, states that salt and salt-petre must never be used together, as he asserts that in this case aqua regia would be formedand the gold dissolved. Agricola, however, apparently found no such difficulty. As tothe other ingredients, apart from nitre, salt, vitriol, and brickdust, they can have been of nouse. Agricola himself points out that ingredients of “ metallic origin ” corrupt the gold and thatbrickdust and common salt are sufficient. In a description of this process in the ProbierbuchleM(p. 58), no nitre is mentioned. This booklet does mention the recovery of the silver fromthe cement by amalgamation with mercury—the earliest mention of silver amalgamation.

19 While a substance which we now know to be natural zinc sulphate was known toAgricola (see note n, p. 572), it is hardly possible that it is referred to here. If green vitriolbe dehydrated and powdered, it is white.