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 



