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 of 
gas possessed any sensible power of intercepting the 
thermal rays from the cube, one face of the pile being de- 
prived of the 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. 
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 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 
