330 
PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
myself the heat impinging on the annular space and diaphragm, reverberated back to 
the end T of the experimental tube, reflected from the annular space and diaphragm at 
that end, and thus in part sent back to the pile. Such heat, instead of passing once 
through the vapour, would pass through it three times, and if it formed a sensible part 
of the total heat, might make vapour absorption appear greater than liquid absorption. 
I had the tube dismounted, and the annular spaces and diaphragms carefully coated 
with lampblack. Remounting the tube and measuring once more the vapour absorp¬ 
tion, it was found to be 
32‘4 per cent. 
This was the mean of five concurrent series of observations, in which every care was 
taken to ensure exactitude. Lest the total heat should vary, during the execution of 
a series, it was taken at the conclusion of every individual experiment. 
The absorption of sulphuric ether, acting as a liquid, was next determined. The 
mean of three series of experiments, two of which yielded identical results, and the 
third of which differred only 0*7 per cent, from the others, made the absorption of the 
liquid ether to be 
32*9 per cent. 
which, I need not say, is surprisingly close to the vapour absorption, differing there¬ 
from by only 0*5 per cent. 
Informed by experiment that the heat from the incandescent platinum spiral was 
more powerfully absorbed than that of the lime light, I thought it worth while to 
inquire whether the liquid followed its vapour in absorptive energy, when the quality 
of the heat was changed. On Monday the 31st, accordingly, the rays from the spiral 
being rendered parallel by a rocksalt lens, the absorption of sulphuric ether was 
determined and found to be 
66*7 per cent., 
while the absorption of the liquid was 
67*2 per cent. 
which again gives a difference of only 0*5 per cent. 
On another occasion I found the absorption by sulphuric ether to be : 
Vapour . . . . 71 per cent. 
Liquid .... 70 „ 
On the 1st November I checked the result obtained with the lime light and mirror, 
by using the lime light and rocksalt lens. Here are the absorptions of the vapour and 
liquid respectively :— 
Vapour . . . . 33*3 per cent. 
Liquid .... 33*3 „ 
The absorptions are identical; while the result agrees closely with that obtained 
with the concave reflector. 
