PHYSICAL BASIS OF SOLAE CHEMISTRY. 383 



declared by the galvanometer. The quantity absorbed 

 by the oxygen under those circumstances was too feeble 

 to affect the galvanometer ; the gas, in fact, proved 

 perfectly transparent to the rays of heat, It had but 

 a feeble power of radiation : it had an equally feeble 

 power of absorption. 



The pile remaining in its position, a sheet of olefiant 

 gas was caused to issue from the same slit as that through 

 which the oxygen had passed. No one present could 

 see the gas ; it was quite invisible, the light went 

 through it as freely as through oxygen or air ; but its 

 effect upon the thermal rays emanating from the cube 

 was what might be expected from a sheet of metal. A 

 quantity so large was cut off, that the needle of the 

 galvanometer, promptly quitting the zero line, moved 

 with energy to its stops. Thus the olefiant gas, so light 

 and clear and pervious to luminous rays, was proved to 

 be a most potent destroyer of the rays emanating from 

 an obscure source. The reciprocity of action established 

 in the case of oxygen comes out here ; the good radiator 

 is found by this experiment to be the good absorber. 



This result, now exhibited before a public audience 

 for the first time, was typical of what had been obtained 

 with gases generally. Going through the entire list of 

 gases and vapours in this way, we find radiation and 

 absorption to be as rigidly associated as positive and 

 negative in electricity, or as north and south polarity 

 in magnetism. So that if we make the number which 

 expresses the absorptive power the numerator of a 

 fraction, and that which expresses its radiative power 

 the denominator, the result would be, that on account 

 of the numerator and denominator varying in the same 

 proportion, the value of that fraction would always 

 remain the same, whatever might be the gas or vapour 

 experimented with. 



