b

d

e

FIG. 54.

k

m

n

by fig. 56, where A is the fire-clay support, and the rest of the pieces are those which are shown at fig. 54, and described at the letters placed against each of them in this figure. It is evident a that the application of this furnace to crucibles of different sizes depends upon the proper choice of the cylinders here marked i and e. Of course there is only a limited choice of crucibles. suitable for such operations. Three inches is the extreme width between the furnace walls of any of the pieces in fig. 54, from a to g, and 9 though larger cylinders could be used, such as i to o, it must be remembered that the flame of a lamp without blast has only a limited power, and that although a given flame will fuse 1,000 grains of carbonate of soda in a platinum crucible, it may only heat to a moderate redness a large clay crucible. Yet, considering that

h

j

low degrees of heat are suitable for many purposes, it is convenient to have the power of readily adjusting a temporary furnace to the bulk of any crucible

which it is desired to heat.

The clay pieces (fig. 54 i to p) are thos that have been expressly designed for the blast oil furnace already described; but these can also be used for spirit and gas furnaces, the respective sizes being chosen in each case

FIG. 55.

according to the size of the crucible that is to be ignited.

FIG. 56.

In respect to the means of supporting a crucible, it has been shown that clay trivets with a wide flange, namely, the 6-inch trivets fig. 54 p, will support a crucible containing 5 lbs. of iron until that quantity of iron is melted, even under the operations of a blast: so that it is evident that this method of supporting a crucible in a gas flame may be always depended upon when no blowing-machine is employed. The discovery of the fact that a trivet of fire-clay of the form of fig. 55 could sustain a crucible bearing 5 lbs. of cast iron, until that quantity of iron has melted under the action of a blast, induced Mr. Griffin to make some experiments on the joint use of a small blast gas-burner and the small fireclay cylinders that are here described, and these experiments led to the construction of the Miniature Blast Gas Furnace already described, an apparatus that justifies the recommendation that has been given of the use of these cylinders; for in the miniature blast gas furnace, the chemist has an instrument which possesses great power in a small compass, and convenient form, the cost of which is a trifle, and which by the addition or exchange of a few fire-clay cylinders can be modified to suit a great variety of operations at high temperatures.

g

upper part of the jacket serve to support the vessel that

contains the liquid that is to be boiled or evaporated. A porcelain basin of 16 or 18 inches in diameter can be thus supported. It is important to allow between the jacket c and the evaporating basin plenty of space for the escape of the heated air, which ascends from the interior of the furnace. When the evaporating basin is of small diameter, it may be supported on iron triangles, placed in the furnace c. The section shows that around the vertical tube of the gas-burner a there is in the bottom of the furnace e a circular opening which is of 2 inches diameter, and through which air passes freely, partly to feed the flame and partly to be heated by the flame and be directed upwards in a continuous current upon the lower surface of the basin that is to be heated. The flame within the furnace burns steadily. No side currents of air agitate it. No part of it touches the basin, which should receive its heat solely from the mass of ascending hot air. The gas-burner thus arranged and supplied by a gas pipe of 4-inch bore, burns about 3 cubic feet of gas in an hour, and the flame which it produces, acting upon water contained in an open porcelain evaporating basin, will heat from 60° to 212° F.

and when the water boils it is driven off in steam at the rate of more than a gallon of water per hour. The method is consequently applicable to distillation on a small scale, and to numerous other laboratory operations.

LUTES AND CEMENTS.-It may be as well to mention in this part of the work the various lutes and cements which may be employed, either in fire operations or in making good joints in experiments with gases or liquids. The following are the principal kinds: The best fire lute is that described by Mr. Parker, and is composed of good clay two parts, sharp washed sand eight parts, horse-dung one part. These materials are to be intimately mixed; and afterwards the whole is to be thoroughly tempered like mortar. Mr. Watt's fire lute is an excellent one, but is more expensive. It is made of finely-powdered Cornish (porcelain) clay mixed to

the consistence of thick paint with a solution of borax in the proportion of 2 ounces of borax to a pint of hot water.

FAT LUTE is prepared by mixing fine clay, in a fine powder, with drying oil, so that the mixture may form a ductile paste. When this paste is used the part to which it is applied ought to be very clean and dry, otherwise it will not adhere. Glazier's putty is very similar to this.

ROMAN CEMENT. This must be kept in well-closed vessels, and not moistened until the instant it is required for use. PLASTER OF PARIS.-This is mixed with water, milk, or weak glue, or starch water.

These three lutes stand a dull red heat: the two latter may be rendered perfectly impermeable to gaseous bodies by being smeared over with oil, or a mixture of oil and wax.

LINSEED OR ALMOND MEAL, mixed to the consistence of a paste with water, milk, lime-water, or starch paste. This lute is very manageable and impermeable, but does not withstand a heat greater than about 500° F.

LIME AND EGG LUTE.--If just the sufficient quantity of water be added to quick lime to reduce it to a dry powder and that is mixed well and rapidly with white of egg diluted with its own volume of water, and the mixture spread immediately on strips of linen and applied to the part, then powered with quick lime, it forms a good cement. Instead of white of egg, lime and cheese may be used, or lime with weak glue water. This lute dries very rapidly, becoming very hard and adhering strongly to glass; but its great inconvenience is the want of flexibility.

WHITE LEAD MIXED WITH OIL.-If this mixture be spread upon strips of linen, or bundles of tow, it acts much in the same manner as the lime lutes.

YELLOW WAX is often used as a lute, but it becomes very brittle at a low temperature. It may be rendered less brittle, and at the same time more fusible, by an admixture of oneeighth crude turpentine.

SOFT CEMENT is prepared by fusing yellow wax with half its weight of crude turpentine and a little Venetian red in order to colour it. It is very flexible, and takes any desired form under the pressure of the fingers.

WATERPROOF CEMENT.-Mr. Edmund Davy, F.R.S., has described a cement made by melting in a saucepan, two parts by weight of common pitch, and adding to it one part by weight of gutta percha, stirring and mixing them well together until they were completely incorporated with or united with each other. The mixture then formed a homogeneous fluid which may be used in this state for many purposes, and is remarkable on account of the facility and tenacity with which it adheres to metals, stones, glass. It may be poured into a large basin of cold water, in a thinner or thicker stream, or as a cake. In this state, while warm, it is quite soft, but may be soon taken up out of the water and drawn out into longer or pressed into shorter pieces, or cut or twisted into fragments, which may again be readily reunited by pressure. When the cement is cold, or before, it may be removed from the water and wiped dry, when it is fit for use. It is of a black colour; when cold, it is hard. It is not brittle, but has some degree of elasticity, which is increased by a slight increase of heat. It appears to be not so tough as gutta percha but more elastic. Its tenacity is very considerable, but inferior to gutta percha. It softens when put into water at about 100° F.; and if the heat is gradually increased it passes through intermediate states of softness, becomes viscous like bird-lime, and may be extended into threads of indefinite length: it remains in this state even when exposed for some time in a crucible, to the heat of boiling water, at 212° F.; when heated to above 100° F. it becomes a thin fluid. Water appears to have no other action upon it but that of softening it when warm or hot, and slowly hardening it when cold. The cement adheres strongly, if pressed on metal or other surfaces, though water be present, provided such surfaces be This cement is applicable to many useful

warm.

purposes. It adheres with great tenacity to metals, wood, stones, glass, porcelain, ivory, leather, parchment, paper, hair, feathers, silk, woollen, cotton, linen fabrics, &c. It is well adapted for glazing windows, or as a cement for aquariums. This cement does not appear to affect water, and it will apparently be found applicable for coating metal tanks;