« PreviousContinue »
The refiners are in total ignorance of the rationale of this process, and I regret that I can only conjecture it, being unacquainted with the matters most commonly combined with the gold of Bhote, or forming its matrices.
Note to the above paper. By J . P.
The process described by Dr. CAMPBELL is precisely that employed throughout India, and no where more frequently than in Calcutta. I took occasion myself to notice it in the Oriental Magazine for June 1827, for the purpose of pointing out a material error in many manuscript copies, as well as in the English translation, of Anon Fazr.’s description of the same operation. As the passage alluded is short, and the work containing it, now out of print, I venture to subjoin the passage:
“ In GLAnwIN’s translation of the Ayeen Akhery, there is an account of the native process for refining gold, in which it is mentioned, that a composition of ‘equal parts of saltpetre and brick‘-dust’ is spread between the plates of gold, which are then heated red hot, &c.
“ As it is well known to chemists, that the ignition of such a mixture would only disengage nitric acid, the very acid which is actually used in the European method of refinage in the humid way, this passage is calculated to mislead even the scientific reader. The mixture really used by the native refiners is composed of equal parts of common salt (muriate of soda) and brick-dust, just in the same way as is practised in Europe, in what is termed the dry method of refinage. The rationale of the process is, that muriatic acid has the power of dissolving silver and copper at a red heat, and the muriates, being volatile, quit the surface of the gold plate as soon as they are formed, giving place to a fresh action from further acid, until the gold is rendered perfectly pure. The muriate of silver is not decomposed, unless some free alkali be present. Now, the nitric acid will quit all its bases at a red heat, and is itself incapable of acting upon silver at that temperature, although it will assist in oxydating copper and other metals; saltpetre is indeed frequently used in purifying silver. There is then evidently some mistake, and if so, is it attributable to the translation? or to the original work, which is so accurate and particular in most of its details P For the purpose of deciding this question, several old manuscript copies of the Ayeen Akhery were examined. In one the expression was simply shoreh, which agreed with the translation. In another it was shoreh i khifll khttm, the saltpetre Qfhalf-burnt bricks :-—at last, in an older manuscript, the true original reading was discovered, which proved to be nimal: shareh, coarse bilter common salt, such as is given to cattle. The ignorance of copyists had imagined perhaps that the word nimalr was redundant, mistaking shoreh for a substantive, as though it were written “ salt of saltpetre,” and nimalr was therefore henceforward omitted. The ease with which the sense of passages in manuscripts may become varied is further evinced hy the second example, where the original plain sentence of ‘ half of coarse salt, and half brick-dust,’ has suffered two metamorphoses, and appears as merely ‘ the nitre of half-burned bricks!
“Perhaps in this place, a brief account of the whole process will not be devoid of interest.
" The gold to he refined, is beaten out into very thin leaves of 5% inches square, each weighing aboutl00 grains. From 100 to 200 of these leaves are piled over one another, being first dipped in a mixture of oil and water, and then smeared over with a composition of three parts of fine old brick-dust, and one part of common salt.
“ A fire of cow-dung is made on the ground, upon which the pile of gold leaves is placed, and it is farther sprinkled with some more of the composition. Around the whole, a dome of cow-dung is raised, (see Pl. XXXV, fig. 5,) to which fire is applied, and the operator fans it with precaution, that the fire may not become too fierce, and melt the gold. The firing is repeated three times, after which the plates are separated and thoroughly washed. If the purification is to be carried further, another charge of the composition is interstratified with the leaves, and three more fires applied. Sometimes even the whole process is repeated three times. Bullion of 22 carats pure, is refined to 23 carats, by the first three heats. After six fires, it become 23 carats. 2% grains pure. The expence of the process is very trifling, and every part of the residue is saleable to the under refiners, who extract the silver and copper.
“ The heat employed, measured by a. pyrometric alloy cupel, was below the melting point of silver."
In publishing Dr. C.uarnnnn's account, I have with permission omitted his reasoning on the rationale of the Nepal process, to make way for a brief notice of some recent observations by the celebrated French chemist BOUSS!NGAULT*, whose experiments have led to a more accurate knowledge of the subject than was before to be met with even in the best works. This chemist had an opportunity of witnessing the art, now so completely exploded and obsolete among Europeans, in the mint of New Granada: “ Certes c’était," he writes, “ une circonstance des plus piquantes, que de me trouver an milieu de cette métallurgie du l6éme siecle, I non-seulement d'observer ces fourneaux compliqués qui rappelaient la philosophic hermétique, mais encore de me rencontrer, scientifiquement parlant, avec des hommes de cetteépoque. On croyait voir des chimistes qui venaient de se réveiller apres avoir dormi pendant trois siecles.”
Instead of beating the gold into fine leaves, as in India, the practice at Santa Fé is to granulate it, and dispose the grains in porous earthen vessels, in alternate layers with a cement made of two parts of brick and one part of sea salt. The layers of cement are an inch thick; each pot holds 10 or 15 lbs. of gold; and the cementation continues from 24 to 36 hours at a cherry-red heat.
To decompose or reduce the silver, which is retained as a chloride in the brick-dust, the cement is trituratcd with mercury and one-tenth of fresh common salt, in a humid state. The muriate of mercury is washed off and an amalgam of silver and mercury left behind, which yields a very pure silver, (known in the Calcutta market as plata pina.)
* In the‘ Annales de Chimie et de Physique, vol. mv. 1833, page 253.
In the process of cementation, it was evident that the silver was converted into a chloride by the action of dry clay and dry salt. Boussrnoauur commenced his inquiries with precisely the same materials, operating on fine gold dust, containing 26 per cent. of silver, and substituting only well baked Cornwall crucibles for the fragile porous ware of the country :—he was surprised however to find that no action whatever now took place, although he maintained his fire for 72 hours! and to the exultation of the natives he was forced to allow the superiority of their old and despised methods.
To ascertain whether air was necessary to ensure success, one slip of silver, weighing 24.6 grs., was cemented in a well-closed crucible, covered with charcoal powder; while another of the same size was merely encased in cement, withouta crucible, so as to favor the access of air. After seven hours, the former had lost only 0.3 gr., while the latterwas reduced in weight to 9.5 grs. The presence of air was thus proved to be indispensable: it remained to examine in what way it acted. Salt by itself may be fu'zed and sublimed in an open silver crucible, without acting upon it in any degree—the volatilization is materially accelerated by a current of hot air, but without any danger of affecting the metal.
Two slips of silver were again prepared, weighing 6.5: one was cemented with a mixture of pure silex and salt; the other with pure alumina and salt. After four hours, under a muflie at a cherry-red heat, the latter had entirely disappeared ; the cement was slightly agglutinated, crystalline, and no longer saline to the taste. The other slip still weighed 4 grs. ; its surface was remarkably crystallized, and covered with a green glass, which adhered strongly. The cement was also completely vitrified, to which circumstance doubtless the had success of the cementation was attributable.
It is known that pure silex has no action whatever on salt when both are dry, but the moment aqueous vapour is introduced, a powerful re-action commences, muriatic acid is disengaged, and silicate of soda remains. In the above experiments then water must have got to the cement even through the heated muflle of the furnace, and it occurred that the success of the Santa Fé cementation might be mainly attributable to the quantity of wet vapours necessarily formed in the combustion of a wood-fire.
To prove whether it was the water contained in the atmosphere, or that supplied by the fuel, that favored the process, Boussrucamxr placed a slip of silver, coated with the cement, in a porcelain tube, heated red, through which he then passed a current of dry air :—the silver remained untouched.
The vapour of water being thus proved to be the principal agent, it seemed evident that the muriatic acid gas must be decomposed at a red heat by silver, although it is generally maintained that this metal exercises no action whatever on the acid even at high temperatures.
To ascertain this point, a slip of silver, rolled in a. spiral, was placed in a porcelain tube passing through the furnace. A current of muriatic acid gas was admitted from one end, passing first through muriate of lime to dry it ; at the other, a curved tube and chamber was fixed, to collect the gas that might be disengaged. At first some hydrogen was given off, but the disengagement soon ceased, and the muriatic acid gas continued to pass without decomposition. On examination, the surface of the silver was found coated with a varnish of chloride, which had prevented the further action of the acid gas. To remedy this evil, the slip was enveloped in alumina :—the action now went on better, though still slowly, and the chloride had penetrated but a little way into the coat of alumina. In the next experiment, salt was added to the clay, bringing it in fact to the composition of the cementation mixture; and now the decomposition proceeded with rapidity, the salt favoring in a singular manner the spreading of the chloride of silver through the porous substance of the alumina; probably owing to a combination between the two chlorides.
I have dwelt at some length on the above series of experiments, because they afford a beautiful application of scientific inquiry to a rude and practical process which has been handed down and imitated from generation to generation, without the least knowledge of the real action of the materials upon one another ; and so apparently simple, that chemists had hitherto neglected to examine it. Yet in this rude result of the experience of ages, what a host of chemical operations are combined, and how necessary is every step of the process :—-the brick-dust for instance answers a double object, first, to decompose the salt, and time cause the disengagement of the acid gas ;——-and secondly, to absorb with the aid of the excess of the salt, remaining undecomposed, the chloride of silver as it is formed, and thus both to keep the surface of the gold free for fresh action, and to prevent the loss of the silver by evaporation, for the chloride is of itself very volatile.—The porous nature of the pounded brick allows the passage and access of the vapours, and thus gives it a preference over unburnt clay for the object in view. Again the wood fuel, or in this country, the cow-dung cake, giving abundance of aqueous fumes, is indispensable to the process, while the small intensity of its combustion affords a regulated heat so as not to endanger the melting of the gold, and its open texture promotes the circulation of the moisture, through the pile
within.—-—Nothing more effectual could -have been contrived with the same degree of simplicity. V
The humid process of refinage has however of late years been brought to such perfection, that it must finally drive the dry process as the field even in India, on account of its vastly superior economy. I have not space here to enter into any particulars of the new method of refining silver and gold by sulphuric acid, but I may remark that according to a recent publication on the subject by GAY Lussac, the refiners of Paris not only charge nothing for refining gold of low qualities, but actually pay a bonus to be allowed the job, returning to the proprietor all the silver contained in it, and paying themselves out of the copper alloy !
The three accompanying figures, Pl. XXXV. figs. l, 2,3,area representation of appearances observed in a schistose rock, which is composed of argillaceous clay and hornblende, They are interesting fortwo reasons; first, because they assist to determine the period at which the rock was formed which, but for the presence of these appearances, and a few indistinct traces of orthocera, would be referred to the primitive era; and secondly, because they appear to constitute a new species of fossil remains. I have only found them in the valley of the Ponar river, a small stream which rises in the mountains between Lohughat and Almorah. The bed of this stream is about 1500 feet above the sea, and is chiefly composed of the rock in which these remains are found. Lofty mountains ascend to the height of some two or three thousand feet on each side of the river: some of these are composed of primitive and others of transition rocks, and the latter are superimposed on the rock in which these fossils occur. Duringahasty survey of the bed of this river, I found the impressions only in the smooth surface of water-worn masses, and from the.g-1-eat size and globular shape of the latter, I was unable to detach any of the fossils with the hammer, and am therefore deprived of the pleasure I should otherwise have had of transmitting a few specimens to the Society. The accompanying drawing was, however, sketched on the spot, and conveys a pretty accurate idea of the appearance of these fossils as they exist in the rock. They never occur straight, being always bent and distorted, and a great number are usually aggregated together in the same stone. The rings are detached and equidistant from each other, and are always about fourteen or fifteen in number,