sents about 195 holes to the square inch of surface. A constant stream of water is introduced into the battery, which, with the violent agitation produced by the motion of the stamps, carries the pulverized ore through the screens out of the battery as fast as it reaches the requisite fineness. The broken ore is usually fed to the batteries by hand, one man being able to tend or feed three or four batteries. It might prove economical to provide the batteries with a self-feeding arrangement— an improvement rarely attempted yet in California, though practiced in Australia and Europe.

The arrangements for extracting and saving the gold from the crushed ore, though varying largely in their details, have certain features always in common; chief among which is the amalgamation of the gold by means of mercury. The crushed ore and water, or the "pulp," as it is called, is led from the batteries through shallow, descending sluices, passing in its way whatever contrivances may be there adopted for saving the gold, being finally discharged as "tailings" from the lower side of the mill. These sluices are from eight to sixteen inches in width, and two or three inches deep, and have an inclination or grade dependent on the degree of fineness of the crushing, the quantity of pulp they are intended to convey, the means employed for saving the gold, etc. There are in general use two prominent modes of amalgamation-the Grass Valley system, so called from its general use in the mills of Grass Valley, and the system of amalgamation in battery.

THE GRASS VALLEY SYSTEM OF AMALGAMATION.

By this plan no mercury is placed in the batteries, the only portion of gold caught there being such as is too coarse to pass the holes of the screen. Of this coarse gold there is, however, always a notable proportion in the Grass Valley ores. In the practice of this method the bottoms of the sluices are covered with coarse woolen blankets, woven for the purpose, over which the pulp flows. These blankets are spread smoothly, and made to overlap each other in such a way as to prevent the pulp from getting beneath them. As the latter flows over them, the heavier particles, which always keep nearest the bottom of the shallow stream, are caught in the meshes of the coarse fabric and there retained, while the lighter portions pass on with the current. But, as the nap of the blankets soon become filled with sand, which, if unremoved, would soon impair, and, if long continued, destroy their efficiency, it is necessary to frequently remove and wash them, after which they are replaced. For this reason the sluices leading from the

batteries are either made double, or three are used for two batteries, so that the pulp from either may be turned into the middle sluice, while the blankets of its own sluice are being washed-an operation that requires to be performed about once every fifteen or twenty minutes. For the purpose of washing, the blankets are placed in a large tub or vat filled with water, where they are thoroughly rinsed, the auriferous sand falling to the bottom. When the blankets have been washed and replaced, the pulp is again turned on, and those of another sluice are subjected to the same operation. The sluices below the blankets are frequently lined with amalgamated copper plates, or provided with mercurial riffles, having also in some cases the pulp conveyed over shaking tables, or subjected to other mechanical treatment, for saving the finer gold before it is finally discharged. All of these contrivances catch some gold, though most of the latter saved below the batteries is caught upon the blankets. The blanket washings are generally rich in gold, and also in metallic sulphurets, when the latter are present in the ore. They are next subjected to amalgamation in order to extract the free gold which they contain, and the sulphurets are afterwards either suffered to escape with the tailings, or are saved and ground with mercury in iron pans, or treated by Plattner's chlorination process. The amalgamation of the blanket washings is sometimes effected by grinding the whole at once in pans with mercury, but more frequently by passing them through Attwood's amalgamator. This machine is used in connection with a short sluice lined with amalgamated copper plates, arranged so as to form a series of little troughs or riffles containing mercury. At the head of this sluice are two or three horizontal semi-cylindrical troughs, of six or eight inches radius, placed parallel to each other transversely across the bottom of the sluice, and partly filled with mercury. The blanket washings are placed in a box or hopper above, and being slowly washed down, are carried over these troughs and the copper plates and riffles below, by a small stream of warm water, a moderate increase of temperature being found to favor the amalgamation. The mercury in these troughs, together with the sand as it passes over them, is kept in a state of constant and brisk agitation by a wooden cylinder revolving in a direction opposite to that of the current, and thickly set with thin blades of iron which dip into the mercury and nearly reach the bottom of the trough.

AMALGAMATION IN BATTERY.

Frequently, when the gold in the ore is fine, and sometimes also when it is coarse, the plan of battery amalgamation is preferred. By this mode mercury is introduced into the battery, a small quantity being sprinkled in upon the feed side at intervals of from half an hour to two hours, as may be needed-the quantity of mercury required in the battery varying with the richness of the ore and the fineness of the gold; the average amount being about an ounce of mercury for every ounce of gold obtainable from the ore. If the gold be very fine, more is needed-in practice, the quantity being judged of by the appearance and consistence of the amalgam formed. The amalgam in the battery should be too hard to be readily impressible with the finger, and yet not so dry as to become brittle, which might cause it to break up and be thrown out in little pellets through the screen. A small portion of the mercury is thrown out, which, with the gold it catches on its way, forms a little ridge of amalgam on the copper plate, generally placed under the lip of the mortar outside the battery. This amalgam should be of such a consistence that an impression can be made upon it with the finger, and yet not too easily. If the amalgam becomes too soft, no more mercury is added till it regains its normal condition; and, on the other hand, if it becomes too dry and hard, the supply is increased until it is brought to the proper consistence.

For the purpose of collecting the amalgam formed in the batteries, the latter are usually partly lined with plates of sheet copper. Upon the surface of these plates the amalgam collects, not in a layer of uniform thickness, but in irregular bunches and little ridges, the position and thickness of which are mainly dependent upon the "swash" produced in the battery by the order in which the stamps fall. The curious effects of this "swash," in determining the distribution of the amalgam upon these plates, is a point worthy of more attention, perhaps, than it has yet received.

Below the batteries come the sluices, with their copper plates, riffles, etc., for saving the gold escaping from the former; these arrangements, differing generally but little from such as are used in the Grass Valley system; the blankets and their accompaniments, however, being but rarely used where amalgamation in battery is practiced.

Various opinions are entertained by metallurgists and millmen as to the efficiency and economy of battery amalgamation; some, who have practiced it for years still adhering to it, satisfied with their experi

ence, and, while it is no doubt open to certain objections, it is preferable to all others. Quartz mills usually run steadily both day and night; where, however, battery amalgamation is practiced it becomes necessary now and then to stop the mill for a "clean up"-that is, to collect the amalgam, which has accumulated in the batteries and on the copper plates. Sometimes the whole mill is stopped for this purpose, while at others, in order to save time, a single battery only is stopped and cleaned up, and then another, and so on, till the whole are thus gone through with. A "run" in a quartz mill varies, according to circumstances, from twenty to sixty days. The amalgam obtained is strained and retorted in the manner already described.

For the purpose of extracting free gold from quartz, the ore is rarely reduced to any finer state of pulverization than is attained by crushing under the stamps with the screens already described. But when auriferous sulphurets are present, sufficiently rich in gold to make its extraction an object, they are frequently subjected to a further process of pulverization and amalgamation. This is effected by grinding them in a flow of water and mercury in an arrastra, Chili mill, or in some of the many patent cast iron pans or grinding mills of recent invention. These pans having first been introduced as a substitute for the German barrel in working the silver ores of Nevada, where they still continue in use, were afterwards employed also for working the gold ores of this State; and, although they may in certain cases be used here to advantage, especially in treating such mercurial residues as may be collected from the various parts of a quartz mill, they are nevertheless gradually going out of use, many millmen having discarded them altogether. For a description of these pans, and further information touching the extraction of gold from the sulphureted ores, Küstel's recently published work on Concentration and Chlorination may be consulted to advantage.

THE MARIPOSA PROCESS.

This process, so called from its having been first introduced at the Benton mill, on the Mariposa estate, consists in reducing the ore to an impalpable powder, by placing it, previously crushed to a coarse sand, together with a quantity of chilled, half-inch cast iron bullets, in a large horizontal revolving cylinder, or cask of wrought iron, thorough pulverization being effected by the friction of the rolling balls. From this "ball grinder," as it is called, the ore is conveyed to a strong airtight iron chamber, where it is subjected to the action of the vapor of

mercury, volatilized by means of superheated steam. When the amalgamation of the gold is supposed to be complete, the apparatus is suffered to cool down, and the pulp having been discharged into a receiver beneath, is then washed upon a long copper shaking table, to collect the amalgam formed. This process, so far as tried, has worked remarkably well, though the question of its general economy can hardly be considered settled.

CONCENTRATION.

The concentration of ores is a subject of importance in California, chiefly in so far as it relates to the separation or extraction of auriferous sulphurets from the mass of ore, of which they usually constitute not more than one or two per cent., the proportion sometimes being much larger. Notwithstanding its great practical importance, the concentration of sulphurets has hitherto received but comparatively little attention in California. At Grass Valley, and in some other localities, they are saved, to a certain extent, to be subsequently worked by the chlorination, or some other process. For this purpose settling boxes are usually employed, to catch the heavier sand, which is afterwards worked over in a sluice, the cradle or rocker being sometimes used to finish up the work. the work. At Grass Valley, recourse has in a few cases been had to a Cornish round buddle, while a variety of patent concentrators have, to some extent, come into use in different parts of the State.

Of the latter, Hendy's concentrator, in its improved form, is believed to be one of the best. This valuable machine, which is designed for separating the finely comminuted quicksilver, amalgam and gold from the refuse matter and collecting the same, as well as for concentrating and saving the sulphurets, operates through a combination of centrifugal force and gravitation-the only principles, as experience has shown, capable of effectually accomplishing this object. Of late this concentrator has been coming into very general use, it having been introduced into many of the leading mills of Grass Valley, at Virginia City, and elsewhere in the State of Nevada; in Arizona, Mexico, Australia, and most other prominent gold and silver producing countries, giving the most unqualified satisfaction wherever tested. But few of these machines, however, are yet based upon a thorough comprehension of the whole subject, inasmuch as they are incapable of yielding under varying circumstances the best attainable results-this question of the concentration of ores being one beset with many inherent difficulties. The problem to be solved can, indeed, be easily stated, since the object to be obtained consists simply in effecting as complete a sep