RADIATION 45 



Every gas in this table is perfectly transparent to light, 

 that is to say, all waves within the limits of the visible 

 spectrum pass through it without obstruction; but for the 

 waves of slower period, emanating from our heated plate 

 of copper, enormous differences of absorptive power are 

 manifested. These differences illustrate in the most un- 

 expected manner the influence of chemical combination. 

 Thus the elementary gases, oxygen, hydrogen, and nitro- 

 gen, and the mixture atmospheric air, prove to be prac- 

 tical vacua to the rays of heat; for every ray, or, more 

 strictly speaking, for every unit of wave -motion, which 

 any one of them intercepts, perfectly transparent ammonia 

 intercepts 5,460 units, olefiant gas 6,030 units, while sul- 

 phurous acid gas absorbs 6,480 units. What becomes of 

 the wave-motion thus intercepted? It is applied to the 

 heating of the absorbing gas. Through air, oxygen, hy- 

 drogen, and nitrogen, the waves of ether pass without ab- 

 sorption, and these gases are not sensibly changed in tem- 

 perature by the most powerful calorific rays. The position 

 of nitrous oxide in the foregoing table is worthy of par- 

 ticular notice. In this gas we have the same atoms in a 

 state of chemical union that exist uncombined in the at- 

 mosphere; but the absorption of the compound is 1,800 

 times that of air. 



5. Formation of Invisible Foci 



This extraordinary deportment of the elementary gases 

 naturally directed attention to elementary bodies in other 

 states of aggregation. Some of Melloni's results now at- 

 tained a new significance. This celebrated experimenter 

 had found crystals of sulphur to be highly pervious to 

 radiant heat; he had also proved that lamp-black, and 



