Fig. 11. Fig. 12. Fig. 13. Fig. 14. Fig. 15. Fig. 16. Fig. 17. Fig. 18. Fig. 19. Fig. 20.

E. Mitchell sculp. FURNACE.

PLATE CCXXVII.

Fig. 21.

Fig. 23.

Fig. 24.

Fig. 27.

Fig. 25.

Fig. 26.

Fig. 28.

Fig. 30.

Fig. 32.

Fig. 29.

Fig. 31.

Fig. 33.

Fig. 34.

Fig. 35.

Fig. 36.

Fig. 37.

Fig. 38.

View of the whole of the portable apparatus furnace. but most saws are too thick, and when a little used, the teeth get rounded off, which makes them work intolerably slow. I have found by far the best tool to be an old table knife, or rather two of them, worn thin by use, and hacked and jagged as deeply as possible, by striking the edges strongly against each other. These work well and expeditiously, and when they become dull are again roughened by the same simple means. The holes may be drilled with a common gimlet of the largest size, and a little steadiness of hand will easily enable the operator to give them the oblique direction with sufficient accuracy; for much is not required. To make a smooth surface to the parts intended to adapt to each other, first wear them down a little with the soft fire-brick, and then grind them with water on a flat free-stone (a sink-stone for example), and lastly make them entirely fit by rubbing one surface on the other.

"No luting of any kind is ever required; so that the whole may be set up and taken down immediately. Nor is it necessary to bind the pots with metal hoops; for they are thick enough to endure considerable blows without breaking; and yet they will bear, without cracking, to be heated as suddenly and intensely as possible. In short, the black-lead crucible seems to be the best material that could possibly be devised for these purposes.

"The heat which this little furnace will afford is so intense, and so much more than would at first sight be expected from trifling an apparatus, that it was only the accidental fusion of a thick piece of cast iron in it that led us to suspect its power. The utmost heat which we have procured in this furnace has been 167° of a Wedgwood pyrometer piece, which was withdrawn from a very small Hessian crucible when actually sinking down in a state of porcelainous fusion. A steady heat of 150° to 155° may be usually depended on, if the fire be properly managed and the bellows worked with vigour. This is sufficient for most operations in chemistry; and the economy in time and fuel is extreme, since a furnace of the given dimensions will very well raise to the above point of heat in five to ten minutes a Hessian crucible of such a diameter, that the average thickness of burning fuel around its bottom is not more than one inch and a half. A smaller crucible will take a higher heat, but at the risk of its softening and falling in by the weight of the incumbent fuel.

"Coak, or common cinders taken from the fire just when the coal ceases to blaze, and broken into very small pieces, with the dust sifted away, form the best fuel for the highest heat. A light spongy kind of coak, formed of a mixture of coal and charcoal, called Davey's patent coal, also answers extremely well. Charcoal alone has not weight enough, when broken so small as it must be to lie close in this little fire-place, to withstand the force of the blast when very violent. A bit of lighted paper, a handful of the very small charcoal, called in London small coal, and ten or a dozen strokes of the bellows, will kindle the fire in almost as many seconds.

"Various little alterations and arrangements, which will readily occur to the practical chemist, will fit this little apparatus for distillation with an earthen retort, heating a gun-barrel puffed through the fire, bending glass tubes, &c.

"I shall only add, that the dimensions of this furnace were determined merely by the circumstance of having at hand pieces of black-lead pots of this size, so that doubtless they may be varied without any diminution, and probably with some increase of the effect. The fame may be laid of the number of holes; for in another instance four appeared to answer as well as six, with this difference, however, that, by long working, the melted flag of the coak will now and then partially block up one or two of the holes; on which account perhaps the greater number is preferable."