24 
PEOFESSOE TYNDALL ON THE ABSOEPTION ANT) 
equal tensions the vapour of a perfectly pure specimen of the substance last mentioned 
would be found to possess a higher absorptive power than that of ether itself. 
It has been already stated that the tube made use of in these experiments was of 
brass polished within, for the purpose of bringing into clearer light the action of the 
feebler gases and vapours. Once, however, I wished to try the effect of chlorine, and 
with this view admitted a quantity of the gas into the experimental tube. The needle 
was deflected with prompt energy, but on pumping out*, it refused to return to zero. 
To cleanse the tube, dry air was introduced into it ten times in succession; but the 
needle pointed persistently to the 40th degree from zero. The cause of this was easily 
surmised ; the chlorine had attacked the metal and partially destroyed its reflecting 
power ; thus the absorption by the sides of the tube itself cut off an amount of heat 
competent to produce the deflection mentioned above. For subsequent experiments 
the interior of the tube had to be repolished. 
Though no other vapour with which I had experimented produced a permanent 
effect of this kind, it was necessary to be perfectly satisfied that this source of error had 
not vitiated the experiments. To check the results, therefore, I had a length of 2 feet 
of similar brass tube coated carefully on the inside with lampblack, and determined 
by means of it the absorptions of all the vapours which I had previously examined, 
at a common tension of 0‘3 of an inch. A general corroboration was all I sought, and 
I am satisfied that the few discrepancies which the measurements exhibit would disap- 
pear, or be accounted for, in a more careful examination. 
In the following Table the results obtained with the blackened and with the bright 
tubes are placed side by side, the tension in the former being three-tenths, and in the 
latter five-tenths of an inch. 
Table XVIII. 
Absorption. 
Bright tube, 
Blackened tube, 
Absorption -nith 
Vapour. 
0’5 tension. 
0'3 tension. 
bright tube 
proportional to 
Bisulphide of Carbon . 
. 5-0 
21 
23 
Iodide of Methyl . . . 
. 15-8 
60 
71 
Benzol 
. 17-5 
78 
79 
Chloroform 
. 17-5 
89 
79 
Iodide of Ethyl .... 
. 21-5 
94 
97 
Wood-spirit 
. 26-5 
123 
120 
Methylic Alcohol . . 
. 29-0 
133 
131 
Chloride of Amyl . . . 
. 30-0 
137 
135 
Amylene 
. 31-8 
157 
143 
dark fumes rose from the cylinders 
on this occasion 
; a similar effect 
was produced by sulphu- 
retted hydrogen. 
