adheres, and forms cohesive lumps without being sufficiently wet to adhere to the hand. Some advise the addition of gum to the water with which the charcoal is moistened; which addition is, however, useless, water alone being sufficient to give a suitable consistence to the charcoal lining. Crucibles are lined with charcoal by the following method.

The crucible is moistened slightly by being plunged into water and withdrawn as speedily as possible, and about an inch in depth of the charcoal paste, prepared as above, placed in it; the paste is then pressed firmly down by means of a wooden pestle; the blows are to be slight at first, and then increasing in force until it is as firm as possible; another layer is then applied and pressed as before, and the process repeated until the crucible is quite full, taking great care to render all as firm as possible, especially at the sides. In order to make each layer adhere firmly to the other, they must be scratched rather deeply with the point of a knife before a new layer is applied. When the crucible is completely filled, a hole is to be scooped in the charcoal, of about the form of the pot. The sides are then rendered smooth by friction with a glass rod. This is absolutely necessary, so that the metallic globules produced in an assay may not be retained by the asperities of the lining, but may be readily enabled to unite into one button. When a lined charcoal pot is well made, its sides are very smooth and shining. For ordinary use, the lining may be ths of an inch thick at the bottom, and 4th or so at the sides; but in some cases, for instance, when the substance to be fused is capable of filtering through the lining and attacking the pot, as a flux, it

must be at least twice the above thickness in every part.

As before stated, in our article on Reduction, lined crucibles have many advantages over plain crucibles. The lining gives them greater solidity, and prevents a loss of shape when softened; for plain crucibles are always three-fourths empty when their contents are fused; on account of its diminishing much in volume, the pots then have nothing to sustain their sides at the time they soften towards the end of the assay, when the highest temperature is employed. Besides, glassy matters do not penetrate the lining, and exercising no action on it, can be obtained in a state of purity, and their exact weight ascertained; if they were fused in a plain pot, they could not be weighed, because a portion would adhere to the sides, and they would not be pure, having taken up a portion of the crucible in which they were fused.

The lining, too, effects the reduction of certain metallic oxides by cementation, and does away with the necessity of adding powdered charcoal to the body to be reduced. This property is very valuable, because, when an oxide is reduced by mixing it with charcoal, an excess must always be employed, and which excess remains with the metal, and prevents us from obtaining its exact weight.

In certain particular experiments, crucibles are lined with other bodies besides charcoal, such as silica, alumina, magnesia, or chalk, by merely moistening their respective powders with water, and applying the paste as above described for the charcoal. A slight layer of chalk lessens the liability of attack from fused litharge.

Malleable iron crucibles are often very serviceable in assays of fusibility, and of certain seleniurets and sulphurets. They are either made of hammered sheetiron, or by plugging up small iron tubes, as gun-barrels, &c. The latter are preferable, because thick, solid crucibles can be used a number of times, whilst the others are necessarily very thin, and can be used only once. Whenever iron crucibles are employed at a very high temperature, they must be placed in those of earthenware, which protect them from the oxidating action of the air; but when they are not heated above the temperature of a copper assay, they may be used naked, if they are tolerably thick.

For assays at the above temperature, cast iron crucibles may be employed with advantage, instead of wrought iron, because they are very nearly as good, and much less expensive.

CUPELS

are vessels in which the operation termed cupellation is carried on. They are made of such substances as are not acted upon by certain fused oxides, as those of lead or bismuth, and their texture is sufficiently loose to allow those oxides to penetrate their substance readily, and yet be sufficiently strong to bear handling without breaking.

There are a great number of substances of which cupels can be made, which will fulfil all these conditions, but only one is in general use, viz.: the ash of burnt bones. The powdered and sifted ash is washed repeatedly with water, to remove all saline and extraneous soluble matter; and, lastly, dried. It now consists

principally of pure phosphate of lime, with a little carbonate partially decomposed. It is sometimes made. into a paste with water, but I have found beer to answer much better. The following are the proportions I employ: 4 lbs. of bone-ash, and lb. of beer. The above mixture is just sufficiently moist to adhere strongly when well pressed, but not so moist as to adhere to the finger or the mould employed to fashion the cupels. The mould consists of two pieces, one a ring, having a conical opening; the other, a pestle, having a hemispherical end fitting the larger opening of the ring. In order to mould the cupels, proceed as follows: Fill the ring with the composition, then place the pestle upon it, and force it down as much as possible; by this means, the moistened bone-ash will become hardened, and take the form of the pestle; the latter must then be forced as much as possible, by repeated blows from a hammer, until quite home. It is then to be turned lightly round, so as to smooth the inner surface of the cupel, and withdrawn; the cupel is removed from the mould by a gentle pressure on the narrowest end. When in this state, the cupel must be dried gently by a stove; and lastly, ignited in a muffle, to expel all moisture. It is then ready for use.

There are two or three points to attend to in manufacturing the best cupels. Firstly, the powdered boneash must be of a certain degree of fineness; secondly, the paste must be neither too soft nor too dry; and thirdly, the pressure must be made with a certain degree of force. A coarse powder, only slightly moistened and compressed, furnishes cupels which are very porous, which break on the least pressure, and which

allow small globules of metal to enter into their pores, which is the most serious inconvenience of all.

When, on the contrary, the powder is very fine, the paste very moist and compressed very strongly, the cupels have much solidity, and are not very porous, the fine metal cannot penetrate them, and the operation proceeds very slowly; besides, the assay is likely to become dulled and incapable of proceeding without a much higher degree of temperature being employed.

METHODS OF MEASURING THE HEAT OF FURNACES.

As much of the accuracy of an assay depends on the temperature at which it is made, and the temperature required varies with the metal, it is very desirable to possess some means of ascertaining the heat of the furnace more accurately than by the eye. There have been instruments devised for this purpose by several parties. The chief inventors are Mr. Wedgwood and the late Professor Daniell, of King's College. The instruments are termed pyrometers, and both of those just mentioned will be described, and their peculiarities and use pointed out, commencing with Wedgwood's.

This pyrometer is constructed on the principle that the purest clay is contracted in proportion to the heat applied to it. This contraction is occasioned by its giving up the water which it holds with great tenacity, by exposure to a high temperature. It is composed of two parts; of a gauge which serves to measure the degrees of heat, and of small pieces of clay which are employed to ascertain the same degrees by their con