RADIATION. 175 



4. Absorption of Radiant Heat by Gases. 



We have now to submit these considerations to the 

 only test by which they can be tried, namely, that of ex- 

 periment, An experiment is well defined as a question put 

 to Nature ; but to avoid the risk of asking amiss we ought 

 to purify the question from all adjuncts which do not neces- 

 sarily belong to it. Matter has been shown to be composed 

 of elementary constituents, by the compounding of which 

 all its varieties are produced. But besides the chemical 

 unions which they form, both elementary and compound 

 bodies can unite in another and less intimate way. By the 

 attraction of cohesion gases and vapors aggregate to liquids 

 and solids, without any change of their chemical nature. 

 We do not yet know how the transmission of radiant heat 

 may be affected 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 pure by 

 liberating the atoms and molecules entirely from the bonds 

 of cohesion, and employing them in the gaseous or vapor- 

 ous 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 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 hinder- 

 ance. The problem before us is to determine whether 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. 



The source of waves which I shall choose for these 



