PROFESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 365 
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. 
§ 13. 
Lampblack, as already stated, is in accord with the undulations of the visible spec- 
trum ; 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 radiation will penetrate the lamp- 
black. 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 transmission of 
41-3 
per cent, of the radiation from the hydrogen-flame. 
Iodine, in a solution sufficiently opaque to cut off the light of our most brilliant 
lamps, transmitted of the heat of the hydrogen-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-salt. I made a few experiments to 
test the diathermancy of the salt, with the following results : — 
For the heat of a hydrogen-flame, the transmission through a perfectly transparent 
plate of rock-salt was 
82 ’3 per cent. 
For a spiral of platinum wire heated to whiteness by an electric current, the trans- 
mission was 
87 per cent. 
For the same spiral lowered to bright redness, the transmission was 
84-4 per cent. 
