Optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration (1906)

HEAT 389 FIG. 215 little tallow to the ends of the rods, will be seen to drop off in succession. They will appear to pass upwards on the screen, unless the erecting prism is used. The conducting power of a metal is projected by construct- ing Despretz' apparatus within the compass of the lantern, as in fig. 215, the bulbs of the thermometers resting at equal distances upon a bar heated at one end. The scales should be of glass. 228. Mechanical and Molecular Motion.—By using a thermo-pile with any suitable galvanometer projection, the heat generated in any body by friction, or in a bullet by per- cussion, or in a closed vessel of air by compression, or the cold produced by rarefaction, can be easily shown. The readiest way of exhibiting the heat of crystallisation is to project the tube of an air-thermometer placed in a vessel of the solution. Badiophony is unfortunately not a projection experiment, though the lantern is used for it; as the effect is perceived only by an individual observer with the telephone. 229. Specific Heat.—The great differences in this are best projected in the following way. Prepare balls or bullets of equal size, about f inch to 1 inch diameter, of iron, tin, and lead; the two latter cast in a mould, and the iron prepared in any way ; or zinc will answer instead of iron and can also be cast. Each ball is furnished with a small wire hook or handle. Prepare also a flat cake of soft wax about ^-in. to \- in. thick and say 6x2.^ inches superficies, in a frame formed by bending round a strip of tin or thin brass. The com- position of the soft wax should be about 4 parts by weight of beeswax and 3 parts of tallow, varied a little by experience as required; and the cake is best formed on the surface of