RADIATION. 27 
ating the atoms and molecules entirely from the bonds of 
cohesion, and employing them in the gaseous or vaporous 
form. 
Let us endeavor to obtain a perfectly clear mental image 
of the problem now before us. Limiting in the first place 
our inquiries to the phenomena of absorption, we have to 
picture a succession of waves issuing from a radiant source 
and passing th rough a gas; some of them striking against 
the gaseous molecules and yielding up their motion to the 
latter; others gliding round the molecules, or passing 
through the intermolecular spaces without apparent hin- 
drance. The problem before us is to determine whetlver 
such free molecules have any power whatever to stop the 
waves of heat; and if so, whether different molecules 
possess this power in different degrees. 
In examining the problem let us fall back upon an actual 
piece of work, choosing as the source of our heat waves a 
plate of copper, against the back of which a steady sheet 
of flame is permitted to play. On emerging from the cop- 
per, the waves, in the first instance, pass through a space 
devoid of air, and then enter a hollow glass cylinder, three 
feet long and three inches wide. The two ends of this 
cylinder are stopped by two plates of rock-salt, a solid sub- 
stance which offers a scarcely sensible obstacle to the pas- 
sage of the calorific waves. After passing through the tube, 
the radiant heat falls upon the anterior face of a thermo- 
electric pile,* which instantly converts the heat into an 
electric current. This current conducted round a magnetic 
needle deflects it, and the magnitude of the deflection is a 
measure of the heat falling upon the pile. This famous 
instrument, and not an ordinary thermometer, is what we 
shall use in these inquiries, but we shall use it in a some- 
what novel way. As long as the two opposite faces of the 
thermo-electric pile are kept at the same temperature, no 
matter how high that may be, there is no current gener- 
ated. The current is a consequence of a difference of tem- 
perature between- the two opposite faces of the pile. 
Hence, if after the anterior face has received the heat from 
our radiating source, a second source, which we may call 
the compensating source, be permitted to radiate against 
the posterior face, this latter radiation will tend to neu- 
* In the Appendix to the first chapter of " Heat as a Mode of Mo- 
tion," the construction of the thermo-electric pile is fully explained. 
