of the assay be to ascertain the average value of a mineral lode or heap of ore, then the assayer must proceed differently. The portion experimented upon must truly represent, in the respective amounts of its valuable material, gangue, quartz, and earthy matters, the great bulk of that of which it professes to be a sample; and this having been secured, the whole must be carefully powdered and passed through fine sieves, taking care that every portion of the mineral goes through. If this be not attended to it will frequently happen that the few grains left out are sufficient to vitiate the whole assay this is especially liable to be the case when examining ores, the valuable ingredients of which are of a ductile or malleable nature, such as auriferous quartz. In this case it frequently happens that the great bulk of gold exists in the form of one or two small pieces, and these being flattened and beaten out in the operation of powdering will almost certainly be left upon the sieve. In cases like this it is better to collect and assay such pieces separately, and estimate their proportion to the whole weight of the sample, than to attempt to powder and distribute them uniformly.

The ore must always be reduced to a pulverulent form, more or less fine, according to the nature of the chemical operation or assay to which it is to be further subjected. This division is effected by means of the anvil, hammer, pestle and mortar, sieve, method of decantation, or other means generally in use for the preparation of any fine powder. The actual process to be adopted must vary according to the nature of the different bodies under examination. In some cases simple crushing is sufficient; in others the ore will have to be pounded in a mortar; whilst occasionally it is necessary to reduce it to the very highest degree of fineness by elutriation. There is also another operation, which is as strictly mechanical as are the above, viz. washing, dressing, or vanning a sample of ore, the end and aim of which is to separate, in a suitable vessel, by means of water and difference of specific gravity, the earthy or useless, and, in some cases, objectionable portion, from the heavier metallic and valuable portion. This operation is almost always employed on the larger scale in dressing ores for the smelter.

The tools and materials employed are the anvil (and stand), vice, hammer, files, cold chisel, shears, pestle and mortar, steel crushing mortar, sieve, &c.

THE ANVIL AND STAND (Fig. 1).-The anvil-stand is constructed of stout wood, about two inches in thickness, and forms a cube of about two feet square. It contains three or four drawers, which serve to hold the hammers, cold

chisels, shears, files, &c. which are required in an assay office. In the centre is firmly fixed the anvil, and in one corner a vice may be also firmly secured.

In general the anvil and hammer are employed for the purpose of breaking a small fragment from a mass of ore for examination, or ascertaining whether the button or prill of metal produced in an assay be malleable or otherwise. The anvil is also exceedingly useful as a support for a crucible while breaking it to extract the metallic or other valuable contents.

The anvil is most useful in size when it weighs about 28 lbs. ; but one of 14 lbs. will suffice. By reference to the figure, it will be seen that the anvil recommended is of the shape usually employed by the blacksmith.

The hammers, figs. 2 and 3, of which two are requisite,

FIG. 2.

FIG. 3.

ought to have one end flat and square and the other pick or wedge shaped. The horizontal wedge end of fig. 2 is useful for breaking open crucibles, and detaching small fragments from a specimen of ore the flat end for ascertaining the malleability of buttons of metal. This hammer should weigh about 1 lb. The larger hammer, fig. 3, should weigh about 4 lbs., and is employed for breaking coke sufficiently fine for the use of the furnace, and detaching fragments from refractory minerals, in both of which cases either end may be employed, as may seem most serviceable to the operator. The flat end of this hammer is also used for driving a cold chisel in separating masses of gold, silver, copper, lead, &c. for assay. This hammer has a vertical pick or wedge end.

Very hard and stony materials which have to be broken on the anvil (and all such ought to be so treated) scatter many fragments, to the certain loss of a portion of the substance, and the probable injury of the operator;, this can be prevented by wrapping the mineral in a piece of stout brown paper, or, if necessary, in several folds. The fracture can then be safely attempted.

This latter precaution must be specially taken in fracturing gold quartz, or hard rock containing metallic silver, as the loss of a very minute quantity of metal would involve a considerable error in the result afforded by the assay.

All minerals, however, unless very friable, must be reduced to a moderate size-say that of a walnut-by means of the anvil and hammer, before pulverisation; otherwise, if the reduction be attempted in a mortar, it is nearly certain to be injured; moreover, the operator will find his labours much abridged by using the anvil for this purpose.

The anvil can also be made very serviceable in repointing

worn or burnt-out tongs, &c. It need scarcely be added that it must be placed as far as possible away from bottles or other frangible articles, otherwise accidents are liable to occur by the forcible projection of fragments of crucibles, stones, &c.

The cold chisel (fig. 4) is employed for cutting off metallic masses for assay. It should be five or six inches long, and about half an inch wide, which is the best size for

general use. However, for some purposes, as cutting copper and other very tough metals, it is convenient to have a chisel only a quarter of an inch wide, as these metals are so much more difficult to cut, and the small chisel meets with the least resistance.

Small shears (fig. 5) are also exceedingly useful in cutting. off pieces of sheet metal, as lead, for cupellation, scorification, &c.

THE PESTLE AND MORTAR.-Mortars are made of various materials, as cast-iron, bronze, porcelain, agate, &c.; the assayer requires one of cast-iron, one of porcelain, and one of agate.

The iron mortar (fig. 6) ought to be of the capacity of

from three to four pints; the porcelain (Wedgwood ware)

(fig. 7) may contain about two pints. The ease with which a mortar may be used, depends much upon its form; and opinion is greatly divided on this subject. Faraday * says that the pestle should be strong, and the size of its superior part such as may be sufficient to allow of its being grasped firmly in the hand, and below to permit a considerable grinding surface to come in contact with the mortar. Its diameter in the lower part may be about one-third or onefourth of the upper diameter of the mortar. The curve at the bottom should be of shorter radius than the curve of the mortar, that it may not touch the mortar in more than one part, whilst at the same time the interval around may gradually increase, though not too rapidly, towards the upper part of the pestle.

The bottoms of all mortars ought to be of considerable thickness, in order to withstand the smart blows they will occasionally have to receive.

Berzelius recommended (and I have found it extremely serviceable) a mass of pumice-stone for cleansing porcelain mortars. It is used with water as a pestle, and in course of time will be worn to the shape of the mortar; its action will then be more speedy.

Iron mortars can be best cleaned by friction, with a little fine sharp sand, if the ordinary process of washing be not sufficient to remove the adhering substance. Great care must be taken to perfectly dry mortars, especially those of iron, otherwise they will become rusted, and the rust so formed will contaminate the substances pulverised in them.

The iron mortar is principally of use in the reduction of the masses of mineral (broken on the anvil, as before described) to a state of coarse powder, in order to render the substance more readily capable of pulverisation, strictly so called. In the use of the iron mortar, all friction with the pestle ought to be avoided, and the body within it must be struck repeatedly and lightly, in a vertical direction, taking care to strike the large pieces, so that all may be equally reduced. This can be carried on until the whole is about the

* Chemical Manipulation, p. 149.