ture producible increases progressively. In a blast furnace, the bellows can be weighted and worked as heavily as possible at once, and by opening all the apertures for receiving air, the maximum temperature can be produced more rapidly than in a wind furnace; but this is of little use, because as heat passes very slowly through the substance of a crucible, when the object is to fuse its contents it must be heated gradually, so as to avoid running the risk of softening the crucible before its contents are acted upon, or even scarcely made warm.

Wind furnaces are, however, much more serviceable and economical than blast, because they work themselves, and do not require the service of a man to attend to the bellows. A blast furnace is used in a laboratory, in certain cases; for instance, when a single crucible has to be submitted to an intense heat, and when the furnace is small, and the bellows large, in which case the operation resembles a blow-pipe assay.

In whatever manner the air is introduced into any kind. of furnace, either wind or blast, it is evident that the quantity of heat developed in equal-sized furnaces depends upon the quantity of air introduced in the same time: but the degree of temperature is not the same in different parts of the furnace, and the distribution of heat varies according to the manner in which the air is introduced into the midst of the fuel. The side over which the air passes, is kept cold by the current, on which account fire-bars last a long time without becoming oxidised, but the heat rapidly augments up to a certain distance from the bars, at which place it arrives at its maximum; above that it diminishes rapidly, because the air is nearly deprived of its oxygen. Experiment has proved that this maximum is about 2 to 3 inches above the bars or tuyeres.

In common wind furnaces the air enters through the spaces between the horizontal bars which form the bottom of the furnace, and the crucibles are placed on a stand which rests on these bars. By this means the lower and centre part of the crucibles, in which parts the matter to be fused is placed, are exactly situated in the maximum of heat, but the stand being constantly kept cold, by the contact of a current of air,

establishes a continual draining or carrying away of heat from the interior of the crucible outwards, so that the substance submitted to assay can only arrive at the maximum temperature after a length of time, and the maximum then is always inferior to that in the mass of fuel. It is on this account that assays in a blast or wind furnace generally occupy from one hour to two hours. The author has found that the time may be reduced to half that just stated, if a good solid foundation of fuel be made, and the crucible placed on that, and well surrounded by coke, constantly kept close to the pot and the sides of the furnace: in this manner the cooling effect of the stand is removed, and the consequent maximum effect of the furnace produced, but then there is danger of the supporting fuel being burnt away from the crucible and the latter getting upset.

OIL AND GAS BLAST FURNACES.

It sometimes happens that metallurgists and assayers have occasion to melt metals at a white heat, but do not wish to heat a large furnace for the purpose. In these cases either the gas or oil furnaces, now to be described, will prove very useful.

OIL FURNACES.—Mr. Charles Griffin, the son of the wellknown chemical instrument maker of that name, described, in the Chemical News,' for January 2, 1864, an oil lamp, which is not only as powerful in action as the best gas furnaces, but almost rivals them in handiness and economy.

DESCRIPTION OF THE APPARATUS.-The oil-lamp furnace is represented in perspective by fig. 30, and in section by fig. 31. It consists of a wick-holder, an oil-reservoir, and a fire-clay furnace; to these must be added a blowing-machine for the supply of atmospheric air.

The oil-reservoir is represented at a, fig. 30; it is made of japanned tin-plate, mounted on iron legs, and fitted with a brass stop-cock and delivery-tube. Its capacity is a little more than a quart. The wick-holder is represented at b, and the upper surface of it by the separate figure c, fig. 32. The wick-holder and the oil-reservoir are consequently

detached. d is a tube which brings oil from the funnel e, and f is a tube to be placed in connection with the blowing apparatus. The wick-holder contains three concentric wicks, placed round the multiple blowpipe c, which is in communication with the blowing tube.

The crucible furnace consists of the following parts, shown in figs. 30 and 31:-g is an iron tripod; h is a flue for collecting and directing the flame. This flue is of such a width, that when the wick-holder, b, is pushed up into it until the top of the wick is level with the top of the clay cone, there remains a clear air-space of about & inch all round

between the wick-holder and the cylindrical walls of the flue. i represents a fire-clay grate having three tongues, shown by i (fig. 32), on its upper surface. These tongues support the crucible, without stopping the rising flame. k is a fire-clay cylinder which rests upon the grate i, and encloses the crucible, forming, in fact, the body of the furnace. Of this piece there are three sizes: the smallest is of 3 inches bore, and works with crucibles that do not exceed 23 inches diameter; a middle size, 4 inches bore, for crucibles not exceeding 33 inches diameter; the largest size, 5 inches bore, for crucibles not exceeding 43 inches

diameter. This piece being heavy, is provided with handles, as represented in p, fig. 32. The walls of the cylinders are from 1 inch to 1 inch thick. is a flat plate of fire7 clay with a hole in the centre, used to cover the cylinder k, so as to act like a reverberatory dome; m is a cover which prevents loss of heat from the crucible by radiation, but gives egress to the gaseous products of the combustion of the oil; n is an extinguisher to put over the wick-holder when an operation is ended; and o is a support for the wick-holder. No chimney is required.

MANAGEMENT OF THE OIL-LAMP FURNACE.-The apparatus is to be arranged for use as it is represented by fig. 30. The

cylinder, k, is to be selected to fit the crucibles, and the crucible of a size to suit the quantity of metal that is to be melted: 1 lb. of iron requires the smallest of the three cylinders, described above; 14 lb. the middle size; 5 lbs. the largest size. The air-way between the crucible and the inner walls of the cylinder should never exceed inch nor be less than inch.

The cotton wicks must be clean, and be trimmed a little below the level of the blowpipe c. If properly managed, they do not readily burn away, but can be used for several fusions. The reservoir should be filled with oil for each

operation. The proper sort of oil for use is the more volatile kind of mineral oil, of the specific gravity of 750, which is now easily procurable at about three shillings per gallon. The variety known by the commercial name of turpenzine answers well. The combustion of a quart of this oil, costing ninepence, gives heat sufficient to melt 5 lbs. of cast iron. Probably the lighter kinds of paraffin oil may be suitable. Liquids of the alcohol class, spirits of wine, and pyroxylic spirit can be used; but they are less effective and more expensive than turpenzine. Care must be taken not to spill the oil on the table or floor, and not to decant it carelessly in the neighbourhood of a light, because atmospheric air strongly charged with the vapour of these light oils is explosive. When the oil is burnt in the furnace in the manner described below, there is no danger. During an operation, a wooden screen, as represented by the dotted lines in fig. 30, should be placed between the oil-reservoir and the furnace, to prevent the vaporisation of the oil by radiant heat. As the wick-holder b, and supply pipe d, contain only about one fluid ounce of oil, the oil must run continuously during a fusion, from the reservoir a into the funnel e, in order that the cotton may be always flooded. The success of the fusion depends upon the due supply of oil, to which point the operator must pay attention. At the commencement of a fusion, the oil must be run from the reservoir until the surface of the oil in the funnel has a diameter of about an inch. The wicks will then be flooded, and a light may be applied, and a gentle blast of air then set on. The oil immediately sinks in the funnel, and the stop-cock must be opened and so regulated as to keep the oil barely visible at the bottom of the funnel. If too much oil is supplied it immediately rises in the funnel, and simultaneously overflows the wick-holder. Too much vapour is then thrown into the furnace, and the heat is immediately lowered, especially at the beginning of an operation, before the fire-clay portions of the furnace are well heated. If, on the contrary, too little oil is supplied, the wicks burn, and the operation is spoilt. The demand of the wick-holder for oil depends upon the condition of the furnace and the character of the fusion in pro