296 EADIATION. 



they foi-ni, both elementary aiid compound bodies can nnite in another and less 

 intimate way. By the attraction of cohesion erases and vapors aggregate to 

 liquids anil solids witliout any change of their chemical nature. We do not yet 

 know how the transmissijon of radiant heat may l)e aftected by the entanglement 

 due to cohesion, and as our object now is to examine the influence of chemical 

 union alone, we shall render our experiments more pm'o by liberating 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 absorp- 

 tion, we have to picture a succession of waves issuing from a radiant source and 

 passing through 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 inter-molecular spaces without apparent hinderance. The 

 problem before us is to determine whether such free molecules have any povrer 

 whatever to stop the waves of heat, and if so, whether different molecules pos- 

 sess this power in different degrees. 



The S(jurce of waves which I shall choose for these experiments is a plate of 

 copper, against the back of which a steady sheet of flame is permitted to play. 

 On emerging from the copper, 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, this being the only solid substance which offers a scarcely sensible 

 obstacle to the passage of the calorific waves. After passing through the tube, 

 the radiant heat falls upon the anterior face of a thermo-electric pile, where it ia 

 instantly aj^plied to the generation of an electric current. This current con- 

 ducted 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 somewhat 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 generated. The current is a consequence of a differ- 

 ence of temperature 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 neutralize the former. 

 When the neutralization is perfect, the magnetic needle connected with the pile 

 is no longer deflected, but points to the zero of the graduated circle over which 

 it hangs. 



And now let us suppose the glass tube, through which pass the waves from 

 the heated plate of copper, to be exliausted by an air-pump, the two sources of 

 heat acting at the same time on the two opposite faces of the pile. Perfectly 

 equal quantities of heat being imparted to the two faces, the needle points to 

 zero. Let the molecules of any gas be now permitted to enter the exhausted 

 tube ; if these m.olecules possess any sensible power of intercepting the calorific 

 waves, the equilibrium previously existing will be destroyed, the compensating 

 source will triumph, and a deflection of the magnetic needle will be the imme- 

 diate consequence. From the deflections thus produced by different gases w'e 

 am readily deduce the relative amounts of wave motion which their molecules 

 intercept. 



In this way the substances mentioned in the following table were examined, 

 a small portion only of each being admitted into the glass tube. The quantity 

 admitted was just sufficient to depress a column of mercury associated with the 

 tube one inch : in othea: words, the gases were examined at a pressure of one- 

 thirtieth of an atmosphere. The numbers in the table express the relative 



