KADIATION. 37 



duced. But, besides the chemical unions which they 

 form, both elementary and compound bodies can unite 

 in another and less intimate way. Gases and vapours 

 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 cohe- 

 sion, and employing them in the gaseous or vaporous form. 



Let us endeavour to obtain a perfectly clear mental 

 image of the problem now before us. Limiting in the 

 first place our enquiries 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 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 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 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, a solid substance which offers a scarcely 

 sensible obstacle to the passage of the calorific waves. 

 After passing through the tube, the radiant heat falls 



