420 CHROMATICS SPECTRA. 



be placed in spherical flasks, they will constitute lenses which will 

 refract the transmitted rays to well defined foci. The focus of the 

 transparent solution will be brilliantly illuminated, but will have 

 little heating power ; that of the opaque solution will be invisible, 

 while gun-cotton placed there will be instantly exploded. Platinum- 

 foil has been raised to a red heat at one of these dark foci. 



654. Selective Radiation and Absorption. 



Radiation of light or heat consists in giving motion to 

 the ether ; absorption consists in taking motion from the 

 ether. Molecules of one kind are able to vibrate at one 

 rate ; those of another kind may be obliged to vibrate at a 

 different rate. The first set of molecules may be able to 

 give to the ether, or take from it, a rate of vibration which, 

 in the ether, constitutes obscure heat. These molecules 

 can absorb or radiate obscure heat. They maybe unable to 

 vibrate at the higher rate which will enable them to absorb 

 or radiate light. They must either transmit or reflect 

 light that falls upon them. In other words, a body ab- 

 sorbs with special energy the kind of rays itself can radiate, 

 both the absorption and the radiation depending upon the 

 possible rate of vibration of the molecules of the body. 



(a.) In the case of gases, the period of molecular vibration is 

 sharply defined. Gaseous molecules, like musical strings, can 

 vibrate at only definite rates. Liquid and solid molecules, like 

 sounding-boards, are able to vibrate at different rates lying between 

 certain fixed limits. These limits depend largely upon the tem- 

 perature. This principle underlies solar, spectrum analysis. 



655. Relation between Radiation and Ab- 

 sorption. Transparent bodies are transparent because 

 the ether-waves which produce or constitute light pass be- 

 tween the molecules of such bodies without having their 

 wave-motion transferred to the molecules. Diathermanous 

 bodies transmit heat freely because the ether-waves which 

 produce or constitute heat pass between the molecules of 



