CONTRIBUTIONS 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 C., 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. Tiie 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 
