PHYSICS OF THE NINETEENTH CENT. HEAT. 509 



a bar of antimony (shaded dark), and joined alternately in the way 

 shown in the diagram. The pile P is constituted of five such series 

 as are represented at A B ; that is, it contains 25 elements. The law 

 according to which an electric current is generated when one side of 

 the pile is warmer than the other will be found in Chapter XX. The 

 faces of the thermo-pile are coated with lamp-black, which more per- 

 fectly than any other substance absorbs all kinds of radiant heat. The 

 first and the last elements of the piles are connected with binding- 

 screws, one of which is seen at M, and by wires from these it is put 

 into connection with a galvanometer (Chap. XX). The pile is generally 

 mounted on an adjustable stand, and a cone of silvered copper is 

 provided for experiments on radiant heat, in order that a greater 

 number of rays may be caused to fall upon the face of the pile by re- 

 flection from the internal surface of the cone. When a pile so fitted 

 up is connected with Thompson's reflecting galvanometer (Chap. XX.), 

 the arrangement becomes an apparatus of the most wonderful delicacy. 

 Thus it will indicate the radiation from the hand held opposite to the 

 cone at the distance of 10 feet or more. 



The results obtained by Leslie, Melloni, and others, established 

 several important laws, among which may be specified that which 

 enunciates the equality of the absorptive and emissive powers of 

 bodies for each kind of heat. Melloni broke ground almost quite 

 new in his researches into the passage of radiant heat through dif- 

 ferent bodies, which he termed diathermancy. Diathermancy expresses 

 the same action with regard to heat as transparency does with regard 

 to light. Melloni used heat from four different sources, viz., a lamp; 

 platinum wire heated in an alcohol-flame ; copper at 400 C. ; and 

 copper at 100 C. The experiments were made by noting the galva- 

 nometrical deflection produced by the heated body at a suitable dis- 

 tance from the pile, and then observing the deflection when a layer 

 of the substance whose diathermancy was to be determined was inter- 

 posed. One of the most notable results was that of all substances, 

 rock-salt was the most pervious to all kinds of heat-rays, permitting in 

 all cases 92 per cent, of the total heat to pass. Glass and Iceland spar, 

 both as transparent to light as rock-salt, entirely stopped the radiation 

 from copper at 100, and allowed only 6 per cent, of that from copper 

 at 400 to pass, while they stopped three-fourths of the radiation from 

 incandescent platinum, and three-fifths of that from the lamp. Alum 

 allowed 9 per cent, of the radiation from the lamp and 2 per cent, of 

 that from the platinum to pass, and stopped all from the other two 

 sources. Ice behaved like alum, except that it permitted only 6 per cent, 

 of the radiation from the lamp and \ per cent, from the incandescent 

 platinum to pass. The few substances here mentioned differ very 

 remarkably as to their diathermancy, although as regards transparency 

 there is no perceptible difference. On the other hand, substances 

 of nearly equal diathermancy may differ very greatly as to their power 



