1864.] Prof. Tyndall — Contributions to Molecular Physics^ 163 



screen, and could be there examined. Falling upon a thermo-electric 

 pile, its presence was shown by the prompt deflection of even a coarse 

 galvanometer. 



What, then, is the physical meaning of opacity and transparency, as 

 regards light and radiant heat ? The luminous rays of the spectrum differ 

 from the non-luminous ones simply in jperiod. The sensation of light is 

 excited by waves of ether shorter and more quickly recurrent than those 

 which fall beyond the extreme red. But why should iodine stop the 

 former, and allow the latter to pass ? The answer to this question, no 

 doubt, is, that the intercepted waves are those whose periods of recurrence 

 coincide with the periods of oscillation possible to the atoms of the dis- 

 solved iodine. The elastic forces which separate these atoms are such as 

 to compel them to vibrate in definite periods, and when these periods syn- 

 chronize with those of the ethereal waves the latter are absorbed. Briefly 

 defined, their transparency in liquids, as well as in gases, is synonymous 

 with discord, while opacity is synonymous with accord between the periods 

 of the waves of ether and those of the molecules of the body on which 

 they impinge. All ordinary transparent and colourless substances owe 

 their transparency to the discord which exists between the oscillating 

 periods of their molecules and those of the waves of the whole visible spec- 

 trum. The general discord of the vibrating periods of the molecules of com- 

 pound bodies with the light-giving waves of the spectrum may be inferred 

 from the prevalence of the property of transparency in compounds, solid, 

 liquid, and gaseous, while their greater harmony with the extra-red periods 

 is to be inferred from their opacity to the extra-red rays. 



Water illustrates this transparency and opacity in the most striking 

 manner. It is highly transparent to the luminous rays, which demon- 

 strates the incompetency of its molecules to oscillate in the periods which 

 excite vision. It is as highly opaque to the extra-red undulations, which 

 proves the synchronism of its periods with those of the longer waves. 



If, then, to the radiation from any source water shows itself to be emi- 

 nently or perfectly opaque, it is a proof that the molecules whence the 

 radiation emanates must oscillate in what may be called extra-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 transmission aug- 

 ments with temperature, we may expect^^the radiation from this flame to 

 be copiously transmitted by the 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 every other liquid examined transmits 

 more or less of the heat, a layer of water of the above thickness is entirely 

 opaque to the radiation from the flame. Thus we establish accord be- 

 tween the periods of the molecules of cold water and those of aqueous 

 vapour at a temperature of 3259° C. But the periods [of water have 

 already been proved to be extra-red ; hence those of the hydrogen flame 



