222 KNOBLAUCH ON RADIANT HEAT. 



We possess two means of judging of the dissimilarity or simi- 

 larity of rays of heat, transmission and absorption. Thus we 

 know that different kinds of heat permeate one and the same 

 diathermanous substance differently (p. 191 and 203), or one 

 and the same substance to an unequal extent (p. 206 to 207) j 

 whilst the same kind of heat does not admit of our recognising 

 any differences in either the one or the other case. 



When there is a choice between the two means, that by trans- 

 mission deserves unqualified preference, because it is a more deli- 

 cate test-method than absorption, I at least have always found 

 that the differences yielded by the transmission of different rays of 

 heat through the same diathermanous media are always greater 

 than those found by the absorptive method, and could often very 

 readily perceive small shadows with the transmission when they 

 were imperceptible by the absorptive process. 



Hence I have also endeavoured to decide the present question 

 by observing whether the heat emitted in the different cases ra- 

 diated through the same diathei-manous bodies in a different or 

 always in the same proportion. 



1. A number of adiathermanous substances were first heated 

 to 212° F. by conduction, by placing them upon metallic cubes, 

 which were kept at this temperature by boiling water. When 

 it is required to examine the heat radiated from different sur- 

 faces as regards its transmission through diathermanous bodies, 

 the same deflection of the thermo-multiplier must first be pro- 

 • duced by each of them before the insertion of the diathermanous 

 media about to be used for experiment between the source of 

 heat and the thermal pile. This object was attained by approxi- 

 mating or removing the foi'mer to such a position that the requi- 

 site deflection of the galvanometer-needle was produced. Expe- 

 riment led to the following result : — When the heat from the 

 bare metallic surface had passed through a diaphragm, and 

 acted upon the pile so as to deflect the needle of the multiplier 

 to 35^, this was found to recede to 10°*25 when red glass 1'5 

 millim. in thickness was introduced behind the perforated screen 

 by the side of the thermoscope. The deviation of 10°'25 was 

 produced by the heat which passed through the glass. The 

 same deflection was obtained when, instead of the metallic sur- 

 face, wood, porcelain, paper, lamp-black, white lead, or any other 

 substance, had radiated upon the instrument. 



The same occurred in all other diathermanous substances. 



