EADIATION. 191 



and heat, and we now know with certainty that the radia- 

 tion of heat and the radiation of light are absolutely 

 identical, and that the only difference between them is in 

 the absorptive power of the substances composing our eyes 

 that they only allow a certain amount of rays to pass ; 

 there are rays not only beyond the red end but also beyond 

 the violet end of the spectrum. Those were first discovered 

 by Bitter, and this was investigated extensively by Professor 

 Stokes in a manner which I hope you will have the pleasure 

 of hearing from his own mouth in one of his lectures in 

 his celebrated experiments on fluorescence. 



Of late years no experiments of great importance have 

 been made adding in any manner to this theory, but the 

 science of radiation has been advancing nevertheless. I 

 will simply allude to two pieces of apparatus here. This 

 is one used by Professor Tyndall in his researches on the 

 radiation of gases, or rather on the absorption of gases ; 

 but since radiation and absorption are equal, it follows 

 that where we have radiation we have an equal amount of 

 absorption. He employed a non-luminous source of heat. 

 He closed his tube with pieces of rock-salt, heat was 

 radiated through the tube, and struck on this thermopile. 

 The thermopile was in connection with a galvanometer, by 

 means of which the effect was measured. In the most 

 delicate experiments it is necessary to heat also the other 

 side of the thermopile, so that the mode of experimenting 

 is this : The tube is exhausted of air entirely, and the 

 current is reduced to zero by placing a vessel of hot 

 water at the other side, so that this side of the 

 thermopile is as much warmed as the other side is by the 

 radiation from the tube. A cube filled with boiling 

 water is placed at the other end of the tube : it absorbs a 

 certain amount of radiant heat, and this difference is 

 shown by a diminution in the temperature of this side of 

 the thermopile, which is indicated by the galvanometer. 

 The most interesting result of this was that apparently 

 gases which are of the most complex molecular structure 

 had the greatest effect in absorbing the red end of the 

 spectrum. Unfortunately in employing aqueous vapour 

 there is a tendency to deposit it on the sides, and you see 

 that not only is the heat radiated directly through this 

 tube, but also to the sides, and receives a number of 



