178 FRAGMENTS OF SCIENCE. 



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 unex- 

 pected manner the influence of chemical combination. Thus 

 the elementary gases, oxygen, hydrogen, and nitrogen, and 

 the mixture atmospheric air, prove to be practical vacua to 

 the rays of heat ; for every ray, or, more strictly speaking, 

 for every unit of wave-motion, which any one of them is 

 competent to intercept, perfectly transparent ammonia in- 

 tercepts 5,460 units, defiant gas 6,030 units, while sulphur- 

 ous 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, hydrogen, and 

 nitrogen, on the contrary, the waves of ether pass without 

 absorption, and these gases are not sensibly changed in 

 temperature by the most powerful calorific rays. The po- 

 sition of nitrous oxide in the foregoing table is worthy of 

 particular notice. In this gas we have the same atoms, in 

 a state of chemical union, that exist uncombined in the 

 atmospheric air; 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 another 

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

 tained a new significance ; for this celebrated experimenter 

 had found crystals of the element sulphur to be highly per- 

 vious to radiant heat ; he had also proved that lamp-black 

 and black glass (which owes its blackness to the element 

 carbon) were to considerable extent transparent to calorific 

 rays of low refrangibility. These facts, harmonizing so 

 strikingly with the deportment of the simple gases, sug- 



