42 FRAGMENTS OF SCIENCE 



not necessarily 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. 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 lib- 

 erating 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 im- 

 age 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 radi- 

 ant source and passing through a gas; some of them strik- 

 ing 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 ap- 

 parent hindrance. 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 mole- 

 cules 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 emerg- 

 ing from the copper, the waves, in the first instance, pass 

 through a space devoid of air, and then enter a hollow 



