PHYSICAL BASTS OF SOLAR CHEMISTRY 349 



prived of the heat thus intercepted, a difference of tem- 

 perature between its two faces would instantly set in, and 

 the result would be declared by the galvanometer. The 

 quantity absorbed by the oxygen under those circum- 

 stances 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 quit- 

 ting the zero line, moved with energy to its stops. Thus 

 the olefiant gas, so light and clear and pervious to lumi- 

 nous 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 vapors in this way, we find radiation and absorption 

 to be as rigidly associated as positive and negative in elec- 

 tricity, or as north and south polarity in magnetism. So 

 that if we make the number which expresses the absorp- 

 tive power the numerator of a fraction, and that which 

 expresses its radiative power the denominator, the result 



