about the size of fine sand; it can then be transferred to the porcelain mortar, where all blows must be carefully abstained from. The process is thus carried on: the pestle is to be pressed upon with a moderate force, and a circular motion given to it, taking care every now and then to lessen and enlarge the circles so as to pass over the whole bottom of the mortar, and ensure the pulverization of the mass of matter submitted to operation. The finer the state of division to which a mineral is reduced, the more accurate and expeditious will be its assay, so that no labour ought to be spared on this point. Pulverization is rendered much easier by operating on a small quantity at one time, and removing it very often from the sides and bottom of the mortar by means of a spatula. This quantity may be somewhat regulated by the hardness or friability of the substance operated on, the harder it is, the less must be taken, and vice versa.

In the use of the iron mortar, fragments are occasionally projected; this may be prevented by covering the upper part of the mortar with a cloth. This applies also to the porcelain mortar, for the dust of some minerals has a disagreeable taste and smell; from this inconvenience, the operator may in some measure protect himself by means of the cloth. Indeed, in some cases, the ambient powder is highly poisonous, and the cloth is not a sufficient protection; under this circumstance, if the body to be pulverized is unacted upon by water, it must be moistened with that fluid, and the resulting mass dried for use.

Some minerals can be pulverized with greater ease if they be ignited and suddenly quenched in cold water;

in this list is the flint and many other silicious matters. In the pulverization of charcoal for assays, it will be found advantageous to ignite it, as hot charcoal is more readily pulverizable than cold.

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Sifting the Sieve.-The operation of sifting is had recourse to when a very fine powder is required, or when a powder whose parts must be equal is needed. A sieve is also very useful for the preparation of coke for the blast furnace. Sieves of various materials and degrees of fineness are necessary. The larger

sieve for the coke may be made of stout iron wire, and have its meshes from 1 inch to 11⁄2 inches square. One or two sieves may be provided, constructed of perforated zinc plate, these are very useful and economical; for very fine powder, a silk, lawn, or muslin sieve is essentially necessary. These latter may be conveniently and cheaply made by the operator. This is a desideratum, for the sharp angular particles of a mineral soon wear them out, yet they are preferable to the metallic sieves, inasmuch as they cannot contaminate anything they are used with.

The following simple contrivance constitutes the sieve I am in the habit of using; I take a common chip pill-box, of two ounce capacity, and strain over the end of it a piece of fine silk or muslin, (as the case may require), which is fastened round the box with string, in the same manner the paper cover is tied over an ordinary jelly-can-the excess of silk or muslin cut off near the string, and the bottom of the box cut out, the sieve is then finished. As they are so easily constructed, it is well to manufacture several of them, and keep one for copper, one for lead, tin, and so on. It is

convenient to close the aperture of the box with the lid when in use, for then none of the larger particles can be thrown over the sides and injure the sieved product.

Where the powder to be sifted is deleterious or offensive to the operator, a sieve termed a drum, or box sieve, may be substituted. It consists essentially of a box in the shape of a drum, covered tightly at both ends with stout leather; the box is divided into two equal compartments by a sieve, which may be separated entirely from either end of the box: the sieve has a ledge which fits the lower compartment, and one fitting the upper, so that it is perfectly close, and when in use, no unpleasant effect can take place. The method of using this sieve is, however, rather peculiar, and requires some practice to fully develop its powers. One side of the under edge must be held by one or both hands, according to its size, whilst the other side rests on a table or bench; a semicircular oscillating motion must now be communicated to it by moving the hands up and down, at the same time they are being alternately brought into approximation with the sides of the operator.

The use of the ordinary sieve is so well known that it does not require description.

The sieve is extremely serviceable in the separation of some ores from their gangues, especially if the latter be very hard and stony. This point must be particularly noted, as it is the cause of much variance between the results of different assayers-for instance, part of the same sample of ore might be sent to two assayers, and the produce made by one might be 8 per cent., and that made by the other, 9, or 94, or in some cases even more. This discrepancy most probably arises from the

cause just mentioned; in the one case, part of the hard gangue might be rejected, and so render the residue richer; and in the other the whole might be pulverized, and the produce would be of course less, and nearer the actual amount contained in the sample submitted to

assay.

A knowledge of this fact is also very useful, but in another point of view: suppose it were wished to separate, in a speedy manner, any friable mineral, such as galena or copper pyrites, as perfectly as possible by mechanical means; we should employ the sieve. The method of operating is as follows: place a small quantity of the mineral in an iron mortar, and strike repeatedly, slight vertical blows. When it is tolerably reduced, sift it; that which passes through is nearly pure mineral, with only a small quantity of matrix, repeat the pounding and sifting processes, until after a few alternations, that which remains on the sieve is nearly pure gangue.

Decantation. This process can only be employed for those bodies which are not acted upon by water, and is effected thus the substance operated on is reduced to the finest possible state of division by means of the foregone processes; it is then mixed with a quantity of water in a glass or other vessel; after a few moments' repose, the supernatant liquid, containing in suspension the finer particles of the pulverized substance, is poured off, and the grosser parts, which have fallen to the bottom of the vessel, are repulverized, and decantation again made use of. By alternating these processes, the finest possible powder may be obtained in a ready manner; it is seldom, however, that a substance is required in such a minute state of division, as produced by this process,

for assaying by the dry way; in the humid method it is occasionally very useful. Decantation is not only employed for the above purpose, but as a ready means of separating a liquid from a precipitate in an assay by the humid method, or in washing a precipitate with a large quantity of water, in order to free it from any adhering impurity which is soluble in that fluid. In certain cases, where the precipitate to be washed is light, the least disturbance of the vessel containing it occasions its distribution in the liquid, and the consequent loss of a portion in decantation; this can be avoided by the employment of the syphon. The operation is then thus conducted. The syphon is filled with water, and the shorter end placed in the liquid, whose transvasion is to be effected, the forefinger of the right hand being, during this time, applied to the longer end of the instrument; it is now to be removed, and the water will flow out until it be level with the immersed end of the syphon. Fresh water can then be added to the precipitate, and the operation of decantation by the syphon carried on as long as requisite.

CHEMICAL OPERATIONS.

Calcination. This operation has for its object the separation of any species of volatile matter, as water, bitumen, arsenic, &c., from a mineral substance; the atmosphere being excluded, or the ignition of any substance, as quartz, &c., previous to its quenching in water. The hydrates of iron, zinc, and all minerals whose matrices are argillaceous, are calcined for the purpose of expelling combined water. Carbonates of lead, iron,