252 FRAGMENTS OF SCIENCE: 



were exactly equal, thus neutralizing each other. The- 

 needle of the galvanometer being at zero, a sheet of oxygen: 

 gas was caused to issue from a slit between one of the- 

 cubes and the adjacent face of the pile. If this sheet off 

 gas possessed any sensible power of intercepting the- 

 thermal rays from the cube, one face of the pile being de- 

 prived of ttie heat thus intercepted, a difference of tempera- 

 ture 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.. 

 Jt 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 tlm 

 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 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 

 vapor experimented with. 



But why should this reciprocity exist? What is the 



