390 FRAGMENTS OF SCIENCE. 



mous with accord, between the periods of the waves of 

 ether and those of the molecules on which they im- 

 pinge. 



According to this view transparent and colourless 

 substances owe their transparency to the dissonance 

 existing between the oscillating periods of their atoms 

 and those of the waves of the whole visible spectrum. 

 From the prevalence of transparency in compound 

 bodies, the general discord of the vibrating periods of 

 their atoms with the light-giving waves of the spec- 

 trum,, may be inferred; while their synchronism with 

 the ultra-red periods is to be inferred from their opac- 

 ity 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 incandescent aqueous vapour, the 

 temperature of which, as calculated by Bunsen, is 

 3259 C., so that, if the penetrative power of radiant 

 heat, as generally supposed, augment with the tem- 

 perature 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 

 examined transmits more or less of the heat, a layer 

 of water of the above thickness is entirely opaque to 



