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BOOK X.
443
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moistened with water, in order that the powerful vapour which arises may be
repelled. When the ingredients have been heated and the ampulla in which they were placed is whitened with moisture, it is heated by a fiercer fire until all the drops have been distilled8. After the furnace has cooled, the aqua is filtered and poured into a small glass ampulla, and into the same is put half a drachma of silver9, which when dissolved makes the turbid aqua clear. This is poured into the ampulla containing all the rest of the aqua, and as soon as the lees have sunk to the bottom the aqua is poured off, removed, and reserved for use. Gold is parted from silver by the following method10. The alloy, with lead
added to it, is first heated in a cupel until all the lead is exhaled, and eight •It is desirable to note the contents of the residues in the retort, for it is our belief
that these are the materials to which the author refers as " lees of the water which separates gold from silver," in many places in Book VII. They would be strange mixtures of sodium, potassium, aluminium sulphates, with silica, brickdust, asbestos, and various proportions of undigested vitriol, salt, saltpetre, alum, iron oxides, etc. Their effect must have been uncertain. Many old German metallurgies also refer to the Todenkopf der Scheidwasser, among them the Probierbüchlein before Agricola, and after him Lazarus Ercker (Beschreibung Allerfürnemsten, etc., Prague, 1574). See also note 16, p. 234. This use of silver could apply to one purpose only, that is, the elimination of minor
amounts of hydrochloric from the nitric acid, the former originating no doubt from the use of salt among the ingredients. The silver was thus converted into a chloride and precipitated. This use of a small amount of silver to purify the nitric acid was made by metallurgists down to fairly recent times. Biringuccio (iv, 2) and Lazarus Ercker (p. 71) both recommend that the silver be dissolved first in a small amount of acid, and the solution poured into the newly-manufactured supply. They both recommend preserving this precipitate and its cupellation after melting with lead—which Agricola apparently overlooked. 10In this description of parting by nitric acid, the author digresses from his main
theme on pages 444 and 445, to explain a method' apparently for small quantities where the silver was precipitated by copper, and to describe another cryptic method of precipitation. These subjects are referred to in notes 11 and 12 below. The method of parting set out here falls into six stages : α-cupellation, &-granulation, c-solution in acid, d-treatment of the gold residues, «-evaporation of the solution, /-reduction of the silver nitrate. For nitric acid parting, bullion must be free from impurities, which cupellation would ensure ; if copper were left in, it would have the effect he mentions if we understand " the silver " separated from the gold soon unites with it again," to mean that the silver unites with the copper, for the copper would go into solution and come down with the silver on evaporation. Agricola does not specifically mention the necessity of an excess of silver in this description, although he does so elsewhere, and states that the ratio must be at least three parts silver to one part gold. The first description of the solution of the silver is clear enough, but that on p. 445 is somewhat difficult to follow, for the author states that the bullion is placed in a retort with the acid, and that distillation is carried on between each additional charge of acid. So far as the arrangement of a receiver might relate to the saving of any acid that came over accidentally in the boiling, it can be understood, but to distill off much acid would soon result in the crystallization of the silver nitrate, which would greatly impede the action of subsequent acid additions, and finally the gold could not be separated from such nitrate in the way described. The explanation may be (apart from incidental evaporation when heating) that the acids used were very weak, and that by the evaporation of a certain amount of water, not only was the acid concentrated, but room was provided for the further charges. The acid in the gold wash-water, mentioned in the following paragraph, was apparently thus concentrated. The " glass " mentioned as being melted with litharge, argols, nitre, etc., was no doubt the silver nitrate. The precipitation of the silver from the solution as a chloride, by the use of salt, so generally used during the 18th and 19th Centuries, was known in Agricola's time, although he does not mention it. It is mentioned in Geber and the Probierbüchlein. The clarity of the latter on the subject is of some interest (p. 34a) : " How to pulverise silver " and again make it into silver. Take the silver and dissolve it in water with the starcken" wasser, aqua fort, and when that is done, take the silver water and pour it into warm salty " water, and immediately the silver settles to the bottom and becomes powder. Let it stand " awhile until it has well settled, then pour away the water from it and dry the settlings, " which will become a powder like ashes. Afterward one can again make it into silver. " Take the powder and put it on a test,· and add thereto the powder from the settlings from " which the aqua forte has been made, and add lead. Then if there is a great deal, blow on |
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