Prof. Tyndall's Contributions to Molecular Physics, 531 



emitted by a flame of alcohol. The case of the carbon is there- 

 fore precisely antithetical to that of the transparent glass — the 

 former transmitting the heat of long period most freely, and the 

 latter transmitting the heat of short period most freely. Hence 

 it follows that the introduction of the platinum wire, by convert- 

 ing the long periods of the flame into short ones, augments the 

 transmission through the transparent glass and selenite, and 

 diminishes it through the black glass and the black mica. 



§ XIII. Radiation of hydrogen-flame through lampblack, iodine, 

 and rock-salt. — Diathermancy of rock-salt examined. 



Lampblack, as already stated, is in accord with the undu- 

 lations of the visible spectrum ; it absorbs them all ; but it is 

 partially transparent to the waves of slow period. As, therefore, 

 the waves issuing from a flame of hydrogen have been proved to 

 be of slow period, we may with probability infer that its radia- 

 tion will penetrate the lampblack. A plate of rock-salt was 

 placed over an oil-lamp until the layer of soot deposited on it 

 was sufficient to intercept the light of the brightest gas-flame. 

 The smoked plate was introduced in the path of the rays from 

 the hydrogen-flame, and its absorption was measured; the plate 

 was then cleansed, and its absorption again determined. The 

 difference of both gave the absorption of the layer of lampblack. 

 The results were as follows : — 



Table XLI. 



Deflection. Absorption. 



Smoked rock-salt . . . 44-2 82*7 



Unsmoked plate . . . 15'8 24*0 



The difference between these gives us the absorption of the 

 lampblack ; it is 58*7 per cent. ; and this corresponds to a trans- 

 mission of 



41*3 per cent. 



of the radiation from the hydrogen-flame. 



Iodine, in a solution sufficiently opake to cut off the light of 

 our most brilliant lamps, transmitted of the heat of the hydro- 

 gen-flame 



99 per cent. 



In experimenting on liquids with heat of slow period, I 

 noticed that the introduction of the empty rock-salt cell caused 

 the needle to move through a much larger arc than when the 

 source was a luminous one. This suggested to me that a greater 

 proportion of the heat of slow period was absorbed by the rock- 



