duced to a very fine powder under the arastras or tahonas, which are called sencillas or de marco, according as they are furnished with two or four blocks of porphyry or basalt (piedras voladoras), which revolve in a circle from 29 to 38 feet in circumference. From 12 to 15 of these arastras, or mills, are generally ranged in a row under one shed; and they are moved by water, or by mules which are relieved every eight hours. One of these machines brays in the space of 24 hours, from 660 to 880lbs. avoirdupois of ores. The moist schlich (lama) which leaves the arastras, is sometimes washed again in cisterns (estanques de deslamar), the construction of which, in the district of mines of Zacatecas, has been recently carried to perfection by M. Garcès. When the ores are very rich, as in the mine of Rayas at Guanaxuato, they are only reduced under the stones of the mills to the size of coarse sand (xalsonte), and they separate by washing the richest metallic grains (polvillos), which are destined for smelting. This very economical operation is called apartar polvillos.

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I have been assured, that when they are about to amalgamate silver ores which are very poor gold, they pour mercury into the vessel or trough, on the bottom of which the stones of the arastras turn the auriferous amalgamation then goes on in proportion as the ore is reduced to powder, the rotatory motion of the piedras voladeras being favourable to the combination of the metals. I had

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no opportunity of seeing this operation, which is not practised at Guanaxuato. In some great amalgamation works of New Spain, the arastras are still unknown; they are contented with the braying of the mazos; and the schlich which comes from under them is passed through sieves (cedazos and tolvas). This comminution is very imperfect; a powder of an unequal and coarse grain amalgamates very ill; and the health of the workmen suffers greatly, in a place where a cloud of metallic dust is perpetually flying about.

The moistened schlich is carried from the mills or arastras, into the court of amalgamation (patio or galera), which is generally paved with flags. The flour is ranged in piles (montones) which contain from 15 to 35 cwt. Forty or fifty of these montones form a torta, by which name they call a heap of moistened schlich, which they leave exposed to the open air, and which is frequently from 60 to 100 feet in breadth, by 20 to 24 inches in thickness. The materials used in amalgamation, which process is generally carried on in a paved court (en patio), are muriate of soda (sal blanca), sulphate of iron and copper (magistral), lime, and vegetable ashes.

It is by the contact of these different substances, namely, moistened ore in powder, mercury, muriate of soda, sulphates of iron and copper, and lime, that the amalgamation of silver, in the process of cold amalgamation (de patio y por cruto), takes place. They begin at first by mixing salt

with the metallic powder, and they stir (repassar) the paste (torta). According to the purity of the salt used, they give each cwt. of schlich, a quantity which varies from two and a half to twentyfour pounds. They leave the mineral mixed with salt (metal ensalmorado) for several days, in order that the latter may dissolve and be equally distributed. If the azoguero judges the metals to be warm (calientes), that is to say in a state of oxidation, and naturally charged either with sulphates of iron and copper which rapidly decompose in the air, or with muriate of silver, he adds lime to cool the mass; and this operation is called curtir los metales con cal. But they use magistral, if the schlich appears too cold (frios); for example, if it proceeds from ores which display great metallic lustre; if it contains sulphate of lead (negrillos agalenados), or pyrites difficult to decompose in the humid air; and this operation is called curtir con magistral. They attribute to the sulphate of iron and copper, the property of heating the mass; and they only consider it as well prepared, when, moistened and held in the hand, it causes a sensation of heat. In this case the sulphuric acid, which is concentrated in the acid sulphate, attracts the water, and, combining with it, gives out caloric.

We have described two processes of chemical preparation of minerals, salting (el ensalmorar), and tanning (curtir) with lime or magistral. After the interval of some days they begin to

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incorporate (incorporar) the mercury with the metallic powder. The quantity of mercury is determined by the quantity of silver which they think will be drawn from the minerals; and they generally employ in the incorporation (en el incorporo) six times the quantity of mercury which the paste contains of silver. They allow from three to four pounds of mercury for half a pound of silver; and with the mercury, or shortly afterwards, they add to the mass, magistral, according to the nature, or (to use the barbarous language of the azogueros) according to the temperature of the minerals (segun los grados de frialdad). They allow from one to seven pounds of magistral to each pound of mercury; and if the mercury assumes a lead colour (color aplomado,) it is a mark that the paste is working, or that the chemical action has begun. To favour this action, and to augment the contact of the substances, they stir the mass (se da repasso) either by causing about twenty horses or mules to run round for several hours, or by setting workmen to tread the schlich, who for whole days go about barefooted in this metallic mud. Every day the azoguero examines the state of the powder; and he makes the trial (la tentadura) in a small wooden trough (aicara), that is to say, he washes a portion of schlich with water, and judges from the appearance of the mercury and the amalgam, if the mass is too cold or too warm. When the mercury takes an ash colour (en lis ce

nicienta), when a very fine gray powder is separated from it which sticks to the fingers, they say the paste is too hot; and they cool it by the addition of lime. But if, on the other hand, the mercury preserves a metallic lustre, if it remains white, and covered with a reddish or gilt pellicle (telilla roxiza o de tornasol morado or en lis dorada); if it does not appear to act upon the mass, the amalgamation is then considered to be too cold, and they endeavour to heat it (calentar) by a mixture of magistral.

In this manner during the space of two, three, and even five months, the paste is balanced between the magistral and the lime, for the effects are very different according to the temperature of the atmosphere, the nature of the ores, and the motion given to the schlich. If they imagine that the action is too strong, and that the mass is working too much, they allow it to repose: or if they wish to accelerate the amalgamation, and increase the heat, they repeat the stirring more frequently, sometimes employing men, and sometimes mules. If the amalgamation is formed too quickly, and appears in the form of small globules, called pasillas or copos, they feed the paste (si ceba la torta) by again adding mercury with a little magistral, and sometimes with salt. When from the exterior characters the azoguero judges that the mercury has united with the whole silver contained in the ores, and that the paste has yielded (ha rendido),