means of sulphuric acid. The bullion from other mills is sold in the crude state, and suffers a discount proportionate to its baseness. Even with the use of sulphate of copper and salt the percentage extracted from low-grade tailings rarely exceeds 50 to 55 per cent. This is due principally, as above stated, to the necessity of a cheap, rapid, and consequently imperfect treatment. The same tailings, however, worked without chemicals, but otherwise in the same manner, would not yield over 20 to 25 per cent. of their value, and in some cases not so much as that. Richer tailings give better results, and are also treated more carefully. In a mill of large capacity, and in good working order, the cost of working does not exceed $3 to $4 per ton. Hence there is still a margin for profit on tailings assaying only $9 to $10 per ton. The pans now almost universally in use in the Washoe tailing-mills have a capacity of from 6 to 8 tons, and the time of working the charge generally does not exceed four hours. More than half an hour, however, is consumed in charging and discharging, in heating the pulps, &c. The actual time of treatment, therefore, is less than three and a half hours. The following table will show the amount extracted from one half hour to one half hour, from a charge of tailings assaying $15.31 per ton, worked in a pan of eight tons' capacity. Value of tailings, $15.31. Assay value of sample from pan, end of first half hour.. Total percentage extracted, 49.30 per cent. The falling off in the yield in the seventh and eighth half hours is due to the insufficiency of chemicals used, their action having spent itself in the earliest part of the treatment. Until the introduction of the use of the sulphate of copper and salt, the working of tailings was confined to rich lots, where a small percentage of the value would still prove remunerative, and to blanketconcentrations. The richness of the latter consisted to a great extent in amalgam, and its successful extraction was consequently an easy matter. 990 Before dismissing the subject of slimes and tailings, it is necessary to refer to certain classes of these ores which do not yield to the action of sulphate of copper and salt to the extent claimed in the foregoing pages. The precise cause of this rebelliousness has never been accurately ascertained, but from the fact that the bullion produced in working them is always of exceeding fineness (1000 and over) even when a large excess of sulphate of copper is used, it is evident that the evil consists in the presence of some substance which decomposes the sulphate of copper, thus producing the same effect as if none were used. This injurious agent is probably an alkali or alkaline earth. It is to be regretted that we have no knowledge of any cheap chemical re agent producing practically the same effect as chloride of copper, and which would not be subject to this decomposition. The following table will show to what extent the treatment of slimes is affected by the elimination of the copper-salt. Five hundred and sixty-two and one-quarter tons of slime from Chollar ore were worked in charges of 2,500 pounds each, with 15 pounds sulphate of copper and 30 pounds salt to the charge. Total amount worked, 5624 tons. Within half an hour of putting the sulphate of copper into the pan, chemical tests could not detect the presence of copper in solution in the pulp, thus proving that it had been almost immediately decomposed. The same phenomenon has presented itself in the working of tailings. Such cases, however, are exceptional, but prove why in certain instances the use of sulphate of copper and salt may not produce the effects claimed for them. Apart from these exceptional cases the beneficial effects of chloride of copper being thoroughly established, the question naturally arises why its use is principally limited to the working of slimes and tailings, and why it is not more generally employed in quartzmills. There is not the slightest doubt that an intelligent use of chemicals would prove highly beneficial in many cases where the old method of treatment without chemicals is adhered to. The ores of the Comstock ledge are particularly docile under their influence. It is true that many mill-men claim to have investigated the matter, and have pronounced against the use of chemicals, but, as has already been stated, their experiments were carried on in such an unfair manner that failure was inevitable. It could not be expected that a few ounces, or even a few pounds, of sulphate of copper and salt would produce appreciable results upon a ton of ore, and yet mill-men were deterred from using larger quantities through the fear of producing baser bullion. It is a common superstition that fine bullion is a guarantee of skillful amalgamation, than which nothing is less true. Until this prejudice is eradicated it is useless to look for the employment of sulphate of copper in quartz-mills in quantities sufficient to establish its merits. Another cause of indifference on the part of Washoe mill-men is the fact that they generally find no difficulty in returning by their present method of working 65 per cent. of the value of the ore, the amount universally guaranteed by customs mills, and are, naturally enough, not disposed to increase the cost of working without corresponding benefit to themselves. The mine-owners also appear to be satisfied with these results, but their apparant indifference to their own interests does not admit of so simple an explanation. The first quartz-mill to adopt the use of sulphate of copper and salt was that of the Meadow Valley Company, near Pioche, Nevada. This mill commenced operations in the summer of 1870. Owing to a misunderstanding only a small supply of sulphate of copper was on hand at the time, and when this was exhausted the difference in percentage extracted was immediately apparent. During the first week's run the yield was equal to 80 per cent. of the value of the ore. When the chemicals were exhausted the yield fell below 40 per cent., only to recover on the receipt of a fresh supply of sulphate of copper. The following table will show the difference in percentage extracted while working with and without chemicals: July and August, 1870-Percentage extracted, 54.40, working threequarters of the time without chemicals. September-64.39, working one-half the time without chemicals. Average assay value of the ore for the year, $143.21. The ores operated upon were very rich. In such cases the policy of working ores with chemicals in preference to roasting may be questioned. The matter must be decided by a careful comparison of the expense and results of the respective methods. It must furthermore be taken into consideration that roasting involves dry crushing, and consequently a decrease in the working capacity of a mill of a given number of stamps. This again necessitates a larger outlay for the erection of more mills in order to crush a sufficient number of tons per diem to render possible the payment of any considerable amount in dividends. The expense of building furnaces, &c., must also be taken into consideration. Should, however, the ores prove of so rebellious a nature that the difference in percentage extracted by the roasting process be sufficient to overrule these considerations, then it is evident that the wet process must give way to dry-crushing and roasting. At the Nevada Butte Mill, where this method of working with chemicals was introduced during the past summer, it was absolutely neces sary to find some means of reducing the ore without roasting, as it was not sufficiently rich to admit of the latter method of treatment. Here also it was found that whereas only 35 per cent. of the value could be extracted by quicksilver alone, the use of sulphate of copper and salt increased the value to 75 per cent. and upwards. These ores contained still more lead than those of the Meadow Valley mine, and finally became so base as practically to belong to the class of smelting ores, causing an enormous loss of quicksilver, and rendering their beneficiation by the amalgamation process an impossibility. The enterprise had therefore to be abandoned. The only chemicals thus far known to be applicable to the amalgamation process are salts of copper. A great benefit would be conferred upon the mining public by the discovery of other chemicals equally effective, but less expensive, and not so subject to decomposition by the iron of the pan or by the ore itself. H. Ex. 211. -28 CHAPTER XIV. THE TREATMENT OF ORES OF NATIVE SILVER IN CHI- The following account was written by H. B. Cornwall, E. M., of the School of Mines of Columbia College, New York, and is to be published in April in the columns of the Engineering and Mining Journal. It furnishes an interesting view of a remarkable industry. Several districts in Mexico yield considerable quantities of native silver, but nowhere does this class of ore occur so abundantly as at Batopilas, in southern Chihuahua, and the neighboring country. Batopilas especially has become famous for its silver-ore, and the object of this article is to describe the mining and reduction of the native silver-ores of Batopilas, as exemplified by the actual workings of an American company, which has now been in eminently successful operation there for several years. In a future article, the treatment of the sulphureted and other combined ores of silver will be given, with some remarks on mining in Northern Mexico in general. Batopilas is situated in a deep and very narrow valley, or barranca, among the western ranges of the Sierra Madre, in southeastern Chihuahua, about eight days' journey, by mules, from the nearest port on the Gulf of California. The neighboring mountain-ranges show different formations; sometimes the trails lead over trachytic rock, then over granite or diorite, and again over conglomerate and porphyritic formations. All of these may be met within a six hours' ride from Batopilas. The silver-bearing veins, however, are confined to the diorite, and their universal vein-rock or gangue is carbonate of lime, in the white, crystalline form of calcite. Accompanying the native silver, as will be more minutely described presently, are black sulphuret of silver, (plata negra,) ruby silver, (rosiclara,) arsenical iron, (fierro blanco,) galena, (plomo,) and zinc-blende, (copelilla ;) all of which occur, however, in very small quantity. Through the valley runs a river, always supplying a great deal more water than would be necessary to run as many mines as could be worked, and during the rainy season swelling to a torrent. On both sides of the river rise steep mountain ranges, and the silver-veins occur in considerable number in both ranges. The particular mine to be described lies on the east side of the river, and is worked by a main tunnel, entering the mountain some sixty feet above high water, and but a few yards from the river-bank. Three of the veins cut by this tunnel had been worked in old times, and from one of them it is reported that some ten millions of dollars were taken. Probably the report is little if at all exaggerated, for the immense waste-heaps and the size of the workings under ground show how extensively the vein was worked, and if the present richness of the ore extracted from another vein can be taken as a criterion, certainly an immense amount of silver must have been obtained. The veins have the general trend of the mountain-range, although they converge at different points, and some of them cross the valley. All are proved by the present tunnel to be true veins, as they are cut from 300 to 600 feet below the surface-level of the old workings, and show the same character below as above. As is always the case where the silver occurs pure, the ore is not uniformly distributed, but occurs in pockets, and sometimes a vein which has yielded a large bonanza at one time may be worked for several hundred feet without yielding more than a very moderate amount of silver. Still, although the silver is not continuous throughout, yet, in the vein above alluded to, the old workings show a continuous body of ore, varying from 3 to 10 or 12 feet in width, and to a depth (these statements are made from memory, and not from notes) of over 200 feet. The present tunnel has cut eight large veins, all bearing silver, and by judicious working of these veins, following the indications of rich ore, new deposits may be constantly opened, so that once such a district is developed it may be as successfully worked as if there were but one vein with a continuous body of ore. The Mexicans were not prudent miners. If a large and rich deposit was opened they worked it out as rapidly as possible, not providing by dead work on other veins for the time when their bonanza should be exhausted. It has been said that galena and zinc-blende accompanied the silver. These minerals, taken in connection with a lively appearance of the calcite, and the presence of arsenical iron, are the miner's guides. By their occurrence he judges where to look for the silver, and an experienced man can follow them up, until from a merely promising rock he proceeds to the silver itself. The country-rock is diorite, very hard and tough near the mouth of the tunnel, but becoming more tractable further in, and always changing decidedly when near a vein. Occasionally the diorite, in a somewhat altered state, and mixed with calcite, forms the vein-rock, as is the case in the largest and richest vein now worked in the tunnel; but invariably, wherever pay rock occurs, there the silver is found with the above accompanying minerals. As regards the method and expense of mining and reducing the ore, the following facts are given, taken from notes furnished the writer, during a twelvemonths' stay in Batopilas, by the superintendent of the company, who is also the vice-president. The cost of mining per ton, including all dead work, such as running the front of the tunnel, prospecting, &c., is $33; the actual cost of extracting the silver-ore, including necessary drifts, and the work on all the veins yielding silver in paying quantities, is $8 per ton. Hauling to the works, on donkeys, about half a mile, 62 cents per ton. In another article other details of mining expenses and methods will be given. The ore is sorted into three classes: first class, value $2,500 and upwards perton; second class, value $1,000 to $2,500; third class, all under $1,000, averaging perhaps $250. The third-class ore is dumped at the stamps, the better ore is kept in a store-room and weighed out. All the ore is crushed in a battery of three small stamps, weighing about 300 pounds each, with a fall of 9 inches, and a capacity of 8 tons per twenty-four hours. The ore falls through a screen with five-eighth inch slits, and is then charged in the arrastra. The lumps of silver are separated by the screen, cleaned by hand, and, with the larger lumps of pure silver from the mine, refined with the retort silver. The stamps are run by a horizontal water-wheel, which will be described under the arrastra. This latter apparatus it is unnecessary to describe. Suffice it to say that it is a large Mexican arrastra, 9 feet in diameter, with two stone mullers or runners, weighing 600 to 800 pounds each. The wheel that runs it is, however, peculiar. The arrastra is built on the top of a pile of masonry in a deep pit. In the center of the arrastra rises a shaft, revolving on a pivot which rests in a plate raised a little above the bed of the arrastra, and from this shaft horizontal arms project beyond the rim of the arrastra. From |