accordingly. No fire-bars or grates can be used to support crucibles in this gas-furnace, because no material, formed into narrow bars, can sufficiently withstand its powers of fusion and combustion.

A plumbago cylinder, or crucible-jacket, two and a half inches high, two and a-half inches in diameter, and a quarter of an inch thick in the walls. It has several holes of threeeighths of an inch in diameter.

A circular cover or dome, flanged at the bottom, and having a knob or handle at the top. It is pierced with twenty-four holes of a quarter of an inch in diameter, arranged in two rows near the bottom. This dome, when of small size, is made of plumbago; when of large size, of fire-clay.

Plumbago crucibles made with a flange or solid overhanging rim, the use of which is to suspend the crucibles over the gas-burner, by means of the cylinders. When the crucibles are too small to fit the cylinders, a flat plate is filed to fit the crucible, and is then placed on the cylinder, to the diameter of which it is adapted.

Besides these pieces of fire-clay and plumbago, it is necessary to be provided with a strong iron tripod, to sustain the furnace, as represented by c in fig. 36, an iron pan in which to place the furnace, and a quantity of gravel or rounded flints, not less than half an inch, nor more than one inch in diameter. These pebbles form an essential part of this gas furnace.

GAS FURNACE ARRANGED FOR HEATING AT THE TOP.-This gas furnace is exhibited in section by fig. 36: a is the gasburner; b is the support for it when used below the furnace; e is the iron tripod support for the furnace; d, d, are two perforated clay plates adapted to the gas-burner a; e, e, are two clay cylinders. These pieces, a to e, are similar in all the furnaces, and will not require description in each example.

The interior of the furnace, as represented by fig. 36, is built up as follows:-The clay plate, d, is put upon the tripod, c. Over the central hole in d the clay cylinder is placed, and upon that cylinder, two or three of the clay plates. Upon these a porcelain or platinum crucible is

placed. If it is of platinum, a piece of platinum foil may be put between the crucible and the uppermost clay plate, to protect the crucible from contact with particles of iron, or against cementation to the clay. The crucible is to be surrounded by the plumbago jacket. The space between this pile in the centre of the furnace and the two cylinders, e, e, which form the walls of the furnace, is to be filled with flintstones or gravel, washed clean and dried. The stones which answer best are rounded, water-worn pebbles, of half an inch to one inch diameter. These may be piled up to the top edge of the jacket.

It has been found convenient to give the crucible jacket a conical form, the better to adapt it to the usual shape of the crucible. The four figures, 37, 38, 39, 40, show the method of using it so as to make crucibles of different sizes fit the furnace properly.

In these figures a represents a ventilator or hollow support,

G

the sides of which are pierced full of holes. This is placed over the hole in the lower nozzle plate, to permit of the descent and escape of the carbonic acid gas and steam produced by the combustion of the gas in the furnace: b represents a cone open at both ends and pierced full of holes. Its use is to contain the crucible that is to be exposed to heat, as represented by d in figs. 39, 40, 41.

The ventilator and cone together should be equal, or nearly equal, to the height of the body of the furnace: the top of the crucible should be about 2 inches from the flat iron face of the gas-burner, that being in general the place of greatest heat, but this is subject to a variation of inch more or less, according to the supply of gas. The space between the crucible and cone should be about 1 inch; if much wider the heating power of the furnace is diminished. The space between the ventilator and cone, a, b, and the sides of the furnace, must be completely filled by flints of from inch to 1 inch diameter. When the flints split up, the powder produced must be occasionally removed, as it stops the draught of the furnace. In order to raise the crucible to the proper distance from the face of the burner, round clay plates are used: thus, c (fig. 39) shows how to raise a crucible within a cone; and c (fig. 40) shows how a small cone can be raised above the ventilator to the proper height. Different sizes of cones may be used in the same furnace, the cone being chosen in each operation to fit the crucible, the quantity of surrounding pebbles being of no consequence, provided the furnace is filled up to the edge of the cone.

THE PROCESS OF FUSION.-The apparatus being thus arranged, the gas is to be turned on, and lighted; the blowing machine is then to be put into action, and the nozzle of the gas-burner depressed into the central hole of the clay plate d', as shown in fig. 36. The whole force of the blue flame then strikes the crucible; part of it forces its way through the holes in the cone or crucible jacket, and part of it rises and passes over the upper edge of the jacket; after which it finds its way downwards between the pebbles. The carbonic acid gas and the vapour of water which

result from the combustion of the gas, together with the nitrogen of the air and any uncombined oxygen, accompany it. No space being left open for the escape of these gases at the upper end of the furnace, they go downwards through the interstices among the pebbles, and passing through the holes in the ventilator a (fig. 37), and through the central hole in the lower plate a (fig. 36), they escape finally into the air. In this progress, the hot gases give up nearly all their heat to the flint stones. Water and gases escape below at a very moderate temperature: water even runs down in the liquid state, while the stones rapidly acquire a white heat, and if the blast and the supply of gas are continued they retain that white heat for any desired length of time-for hours.

PRECAUTIONS TO BE OBSERVED ON COMMENCING A FUSION.When a furnace and its contents are cold, and a burner is newly lighted, it must not be suddenly plunged into the furnace, and the full heat be applied at once; otherwise the fire is apt to go out, or the crucibles and interior fittings to crack from the too sudden application of a violent heat. It is better to let the flame play a little time into the opening of the furnace, before the burner is thrust closely into its place. The crucibles and furnace fittings should be quite dry when used. It is recommended, after arranging a furnace for a fusion, first to warm it by a large gas-burner, before applying the blast-burner. When the furnace has been warmed the full heat may then be applied safely. At the end of ten minutes after lighting the gas, the crucible, placed in the described circumstances and exposed to the full action of the heat of the gas, and surrounded by substances which are bad conductors of heat, is raised, with the jacket and pebbles around it, to a white heat. The consequence is that the full power of the gas jet is then exerted upon the crucible and its contents, and those effects are produced which will be described presently.

If it is desired to inspect the substance subjected to the action of heat in this furnace, the gas-burner is lifted out, and the crucible is examined through the hole in the clay

plate. To make it possible to inspect substances at a white heat, the view is taken through a piece of dark cobalt blue glass. If the substances submitted to heat suffer no harm from the action of oxygen, it is better to dispense with a crucible cover and to direct the jet of flame directly down upon the substance to be heated. The action is then more rapid. When the burner is taken out, the substance in the crucible can be stirred, if it is considered necessary.

RESULTS. The following experiments will give an idea of the power of a furnace of this description. A common clay crucible, 3 inches high and 3 inches diameter at the mouth, was filled with about 24 ounces of cast iron. It was mounted like fig. 36 in a furnace of 4 inches internal diameter and 8 inches deep. The pebbles were filled in to the edge of the crucible. No crucible cover and no jacket were used. The flame was thrown directly upon the iron. In a short time the iron melted; the oxygen then converted some of the cast iron into magnetic oxide of iron, which formed a thin infusible mass on the surface of the cast iron. At twenty minutes from the lighting of the gas, the furnace was dismounted. The crucible was taken out. A hole was broken by an iron rod in the infusible surface of oxidised iron, and the fused cast iron below it was decanted into a mould, and made a clear casting weighing 20 ounces. In the same small furnace 32 ounces of copper were fused in fifteen minutes. When the furnace is hot, that quantity of copper or cast iron can be fused in ten minutes. In a furnace of the same dimensions, but with a gas-burner having only six instead of sixteen jets, 16 ounces of copper or of cast iron can be completely fused in ten minutes, if the furnace is cold, and in seven minutes if the furnace is hot.

These experiments show that within twenty minutes a heat is producible in this little furnace which is more than sufficient for most assaying or metallurgical operations.

GAS FURNACE HEATED AT THE BOTTOM.-This is exhibited in section by fig. 41.

In this furnace the parts marked a, b, c, d, e, e, are the