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piastres, the profit is 6 per cent.; and it rises to seven per cent. when the produce of the mines is still greater, as was the case during the last twenty years. We shall afterwards

see that the mint of Mexico, and the house of separation (maison du depart), make an annual profit of nearly eight millions of francs.*

The house of separation (casa del apartado) in which is carried on the separation of the gold and the silver, proceeding from the ingots of auriferous silver, formerly belonged to the family of the Marquis de Fagoaga. This important establishment was only annexed to the crown in 1779. The building is very small and very old; and it has latterly been rebuilt, in part, at a greater expence to the government than if its place had been supplied by a new house, not situated in the middle of the town, and in which the acid vapours would have been better directed. Several persons interested in the works of the apartado remaining in their present situation, maintain that the vapours of nitrous acid which are diffused through the most populous quarters of the town, serve to decompose the miasmata of the surrounding lakes and marshes. These

* 326,8301. sterling, Trans.

ideas met with a favourable reception after acid fumigations were used in the hospitals of the Havannah and Vera Cruz.

The casa del apartado contains three sorts of works, which are destined, 1st, to the manufacture of glass; 2nd, to the preparation of nitrous acid; and 3rd, to the separation of the gold and the silver. The processes used in these different works, are as imperfect as the construction of the glass-work furnaces, used for the manufacture of retorts, and the distillation of aqua fortis. The substance of the glass (pasteladura) is composed of 0.46 of quartz, taken from the veins of Tlapujahua, and 0.54 of soda, which the Indians of Xaltocan and the Peňol procure from the incineration of the Sesuvium portulacastrum of several new species of Chenopodium, Atriplex, and Gratiola, which will be described in the Flora Mexicana of M. M. Sesse and Cervantes, and of the Salsola soda of Europe, which is cultivated in the valley of Mexico, both to be eaten as a root, and to be reduced to ashes. This soda of Xaltocan is mixed with a good deal of sulphate of potash and lime; so that the carbonate of soda, which is every where found in efflorescence in clay grounds, would be much better adapted for the manufacture of glass. This pasteladura is not melted in earthen pots as in Europe, but in crucibles

of a very refractive porphyritic rock, procured in a quarry in the vicinity of Pachuaca. More than 15,000 francs are annually consumed in the glass-house furnaces for wood. A retort costs nearly 14 sous at the manufactory, and more than 50,000 are annually broken.

*

The nitrous acid used for the separation, is manufactured by decomposing raw saltpetre, by means of a vitriolic earth (colpa) which contains a mixture of alumine, sulphate of iron, and oxide of red iron. This colpa comes from the environs of Tula, where a mine is worked at the expence of the Farm of Colours. The saltpetre is furnished to the House of Separation, by the royal manufactory of powder. Each retort is charged with eight pounds of colpa, and the same number of pounds of nitrate of impure potash; the distillation lasts from thirty-six to forty hours. The furnaces are round, and unprovided with grates. The nitrous acid which is derived from the decomposition of a saltpetre surcharged with muriate, necessarily contains much muriatic acid, which is carried off by adding nitrate of silver. We may judge of the enormous quantity of muriate of silver obtained in this establishment, if we reflect that there is purified a quantity of nitrous acid, sufficient

* Estanco real de tintes y colores.

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to separate seven thousand marcs of gold per annum. They decompose the muriate of silver by 'fire, melting it with small lead drops. It would be more profitable, undoubtedly, to make use, in the distillation of aqua fortis, of refined, instead of raw saltpetre. They have hitherto followed the slow and laborious method of purifying the acid by nitrate of silver, because the royal establishment of the apartado is under the necessity of buying the saltpetre from the royal manufactory of powder and saltpetre, which will not give out refined saltpetre, under 126 francs the quintal.

The separation of gold and of silver reduced to grains, for the sake of multiplying the points of contact, takes place in glass retorts arranged in long files on hoops, in galleries from five to six metres in length. These galleries are not heated by the same fire, but two or three matrasses form as it were a se

*

parate furnace. The gold which remains at the bottom of the matrass, is cast into ingots of fifty marcs, while the nitrate of silver is decomposed by fire during the distillation in the retorts. This distillation, by which they regain the nitre and acid, is also practised in a gallery, and lasts from 84 to 90 hours.

* From 16 to 19 feet. Trans.

They are obliged to break the retorts to obtain the reduced, and chrystallized silver. They might no doubt be preserved, by precipitating the silver by copper, but it would require another operation to decompose the nitrate of copper, which would succeed to the nitrate of silver. At Mexico, the expence of separation, is reckoned at from two to three reals de plata (from 26 to 39 sous tournois) per marc of gold.

It is surprising that none of the pupils of the school of mines are employed either in the mint, or in the casa del apartado; and yet these great establishments ought to expect useful reforms, from availing themselves of mechanical and chemical knowledge. The mint is also situated in a quarter of the town, where running water might be easily procured to put in motion hydraulical wheels. All the machines are yet very far from the perfection which they have recently attained in England and in France. The ameliorations will be the more advantageous, as the manufacture embraces a prodigious quantity of gold and silver; for the piastres coined at Mexico, may be considered as the materials which maintain the activity of the greatest number of the mints of Europe.

Not only working gold and silver, of which we have already spoken, has been improved

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