TO MOLECULAR PHYSICS. 297 



between the oscillating periods of their atoms and those of 

 the waves of the whole visible spectrum. From the prev- 

 alence of transparency in compound bodies, the general 

 discord of the vibrating periods of their atoms with the 

 light-giving waves of the spectrum may be inferred; while 

 their synchronism with the ultra- red periods is to be infer- 

 red from their opacity to the ultra-red rays. Water 

 illustrates this in a most striking manner. It is highly 

 transparent to the luminous rays, which proves that its 

 atoms do not readily oscillate in the periods which excite 

 vision. It is highly opaque to the ultra-red undulations, 

 which proves the synchronism of its vibrating periods with 

 those of the longer waves. 



If, then, to the radiation from any source water shows 

 itself eminently or perfectly opaque, we may infer that 

 the atoms whence the radiation emanates oscillate in ultra- 

 red periods. Let us apply this test to the radiation from a 

 flame of hydrogen. This flame consists mainly of incan- 

 descent aqueous vapor, the temperature of which, as 

 calculated by Bunsen, is 3,259 degrees 0., so that, if the 

 penetrative power of radiant heat, as generally supposed, 

 augment with the temperature of its source, we may expect 

 the radiation from this flame to be copiously transmitted 

 by water. While, however, a layer of the bisulphide of 

 carbon 0.07 of an inch in thickness transmits 72 per cent, 

 of the incident radiation, and while every other liquid ex- 

 amined transmits more or less of the heat, a layer of water 

 of the above thickness is entirely opaque to the radiation 

 from the hydrogen flame. Thus we establish accord be- 

 tween the periods of the atoms of cold water and those of 

 aqueous vapor at a temperature of 3,259 degrees C. But 

 the periods of water have already been proved to be ultra-red 

 hence those of the hydrogen flame must be sensibly 

 ultra-red also. The absorption by dry air of the heat 

 emitted by a platinum spiral raised to incandescence by 

 electricity is insensible, while that by the ordinary undried 

 air is 6 per cent. Substituting for the platinum spiral a 

 hydrogen flame, the absorption by dry air still remains 

 insensible, while that of the undried air rises to 20 per 

 cent, of the entire radiation. The temperature of the 

 hydrogen flame is, as stated, 3,259 degrees C.; that of the 

 aqueous vapor of the air 20 degrees C. Suppose, then, 

 the temperature of aqueous vapor to rise from 20 degrees 



