PHYSICAL BASIS OF SOLAR CHEMISTRY. 331 



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 ole- 

 fiant 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 ema- 

 nating 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 ob- 

 tained 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 polar- 

 ity 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. 



