Blowpipes may be considered as miniature blast-furnaces. They are little instruments used to force--by means of air blown from the mouth--the flame of a lamp or candle into a jet of flame so fierce that the very highest heat can be produced by it. Various forms of blowpipes are shown in the figures; the common blowpipe, used by gas-fitters, tinmen, &c., is shown at _a_; better blowpipes have generally some reservoir to contain the condensed breath and so prevent it issuing into the jet; the bulb shown at _b_ is for this purpose and also the conical part of _c_. Very good and cheap blowpipes may be made by bending a piece of glass tube into the form shown at _d_, adding a perforated cork and a small piece of bent glass tube fixed as in the figure. The end of the small tube, intended to produce the jet, should be held in the flame of a lamp or gas till it is red hot and turned round all the while; in this way the hole will gradually become smaller as the melted sides collapse, forming a neat round hole about the size to admit a fine needle; with this blowpipe a very great heat can be produced, and it can be easily repaired. The oxy-hydrogen blowpipe is a contrivance for forcing a jet of oxygen and hydrogen gases--mixed together in the proportions in which they form water--through a small orifice and setting fire to it; this produces the very highest heat. Almost any substance can be fused by it, but the experiment should not be made unless with a proper apparatus, as the flame will be sure to run down the tube and explode the mixed gases with dangerous violence. A common flame is merely a cone of vapour burning on the surface where it comes into contact with the air, and therefore gives out but little heat, but when air is forced into it, a small blue cone of solid flame is projected, which gives off more heat than the hollow cone. THERMOMETERS. Illustration: FIG. 1. (‡ BULB AND STALK WITH MERCURY.) Illustration: FIG. 2. (‡ SCALED THERMOMETER.) Illustration: FIG. 3. (‡ REGISTER THERMOMETERS.)