volume at 32°, which is to be deducted; 491 parts of air at 32° become 492 at 33°, and 493 at 34°, and so on; so that at 60° they have increased to 519 parts, and at 80° to 539; so that we have a proportion between the bulks at 60° and at 80°, from which the question may be determined, for : Volume at 80° Volume at 60° 491 + 48 : 491 + 28 :: Cubic inches 100 : Cubic inches or the reverse, supposing it were wished to ascertain the real volume at 60° of 100 cubic inches of gas at 40°. Volume at 40° Volume at 60° Cubic inches 491 8: 491 + 28 :: 100 : Cubic inches 104.008 CORRECTION FOR PRESSURE.-As before stated, the standard pressure is 30 inches of mercury, and the law must be kept in mind that the bulk of a body of gas is inversely proportionate to the weight, and directly proportionate to the pressure; so that if we had 100 cubic inches of air when the barometer was 29 inches it would be as : 30 29 :: 100: 96.6 or if the barometer stood at 31 inches when the 100 cubic inches were measured, it would be as :— 30 31 :: 100: 103.33 : so that the rule is as the mean pressure is to the observed pressure, so is the observed volume to the true volume. The correction for temperature or pressure may be made indiscriminately, the result being the same in either case. CORRECTION FOR MOISTURE.-This correction must be made after the two previous. As before mentioned, the elastic force of the aqueous vapour causes the gas with which it may be mixed to expand, and by reference to tables founded on calculations upon the force of steam at different temperatures, the amount of correction may be easily ascertained. Thus, for 100 cubic inches of a gas saturated with vapour properly corrected to the temperature of 60° and 30 inches pressure, we wish to know the equivalent bulk of the dry gas. The observed volume is partly due to the expansion occasioned by the vapour; and this proportion will be, in proportion to the whole, as the elasticity of the vapour is to the total elasticity; therefore : The volume of the dry gas is therefore : SUBLIMATION. This operation is a kind of distillation in which the product is obtained under the solid form. The apparatus which may be employed for this purpose are tubes, flasks, capsules, or crucibles. Florence flasks are exceedingly useful: they may be sunk in a sand bath, and the sublimed substance received directly into another flask, or by passing through an intermediate tube. Sometimes, however, it is difficult to entirely remove the sublimed substance; and in order to avoid this inconvenience, Dr. Ure has proposed the following very excellent subliming apparatus:-It consists of two metallic or other vessels, one of which is flatter and larger than the other. The substance to be sublimed is placed in the smaller vessel, and its opening is covered by the larger filled with cold water, which may be replaced from time to time as it becomes hot. The sublimed substance is formed on the lower part of the upper vessel. A large platinum crucible, filled with cold water, and placed on the top of a smaller one, answers the purpose of the before-mentioned apparatus very well. SCORIFICATION: CUPELLATION.-These operations will be described under the head of Silver Assay. 52 CHAPTER IV. PRODUCTION AND APPLICATION OF HEAT. FURNACES for assay purposes may be heated either by solid fuel, oil, or gas, and they may be divided into wind and blast furnaces. In the former the fire is urged by the ordinary draught of a chimney; and in the latter by means of bellows or artificial blast. We shall commence with the former, as they are in most common use. They are of various kinds, according to the purposes for which they are required. The three principal kinds are those for fusion, calcination, and cupellation. Coal, coke, and charcoal, are the fuels employed, and the merits of each will be particularly discussed. Blast furnaces are only employed for the purpose of fusion, although their forms are various : charcoal and coke are the fuels most in use, but oil and gas blast furnaces are used in small laboratory operations, and for many purposes they are preferable to other furnaces, on account of their freedom from dust and dirt, and the perfect control the operator possesses over the heat. Furnaces consist of certain essential parts-viz. first, the ash-pit, or part destined to contain the refuse of the combustible employed; secondly, the bars on which the fuel rests; these are sometimes made movable, or fixed to a frame; the former arrangement is more convenient, as it allows clinkers and other refuse matters to be readily removed; thirdly, the body of the furnace in which the heat is produced; and lastly, in wind furnaces, the chimney by which the heated air and gaseous products of combustion are carried off. CALCINING FURNACE.-Calcining furnaces are small and shallow, because a high temperature is not required. They may be made square or circular; the former are most readily constructed, and where many crucibles are to be heated at once, they are preferable to the circular; but the latter give the greatest degree of heat with the least possible consumption of fuel, and are to be preferred on that account where one crucible only is to be ignited. The body of the furnace is best made with good bricks, lined with Welsh lump, fire-bricks, or a mixture of Stourbridge clay and sand. It is also desirable that a plate of iron with a ledge be placed over the upper part of the furnace to protect the brickwork from blows with crucible tongs, &c., and to keep it in its place when disturbed by sudden alterations of temperature. The bars of the furnace may be either in one single piece, or made up of several bars of iron fastened to a frame. They ought to be as far as practical from each other, and must not be too large, although large enough not to bend under the weight of the fuel and crucibles, when they become hot, and they must not be so far removed from each other as to allow the coke or charcoal to fall through easily. Lastly, the more readily the air can find access to the centre of the fuel, the higher will be the temperature produced in the furnace: very simple assays occasionally fail, only because the bars are either too large or too close together. The ASH-PIT is an open space under the bars, which serves as a receptacle for ashes, clinkers, &c., produced during the time the furnace is in use. It should have the same area as the furnace, and be completely open in front, so that the air may have free access: it is well, however, for the sake of economy, to furnish this opening with a hinged door, having a register plate fixed in it, so that the draught may be reduced, or entirely shut off, in order that the fire may be extinguished when desirable, and fuel saved which otherwise would be burnt in waste. CHIMNEY.-Calcining furnaces generally have no fixed chimney, but are covered with a movable one when a greater degree of heat is required. This chimney may be about five feet high, the diameter of the furnace at the bottom, and tapering off to about two-thirds of that diameter at the top. It is made of strong plate iron, furnished with a wooden handle. The lower part is provided with a door, by means of which the interior of the furnace may be examined without disturbing the whole arrangement of the chimney, and consequent cooling of the contents of the furnace. If, during the course of any experiment, noxious or offensive vapours are expected to be given off, the furnace must be so arranged that they may be introduced into a flue, by fastening a piece of iron plate pipe, furnished with an elbow-joint, on to the movable chimney before spoken of. EVAPORATING FURNACES.-The furnaces just described answer exceedingly well in the absence of gas, for heating small flasks, evaporating basins, &c., when surmounted by a tripod stand or sand bath. This is necessary, as many assays by the dry way are preceded or followed by certain operations in the wet way. THE HOOD.-In order to prevent certain gases or vapours from fires, evaporating basins, &c., from entering into the laboratory, a large metal covering, termed a hood, is employed, terminating in a chimney having a good draught. It is best made of plate or galvanised iron. FUSION FURNACE: WIND FURNACE.-The wind furnace, properly so called, is a furnace provided with a chimney, and capable of producing a very high temperature. Wind furnaces are generally square, but if more than four crucibles are to be heated at one time, they may be made rectangular, the chimney being placed at one of the long sides. When the furnace is required to hold but one pot, it may however be made circular. The body of the furnace ought to be made of good bricks, solidly cemented with clay, and bound by strong iron bands. The bricks must be very refractory, and capable of sustaining changes of temperature without cracking. They are ordinarily made with the clay used in the manufacture of crucibles. In some cases bricks are not used for the lining of this kind of furnace; for instance, a mould of wood is placed in the centre and the open space between the surface |