OF GASES AT HTGH EXHAUSTIONS. 
429 
of aqueous vapour was found to have strong action in diminishing the sensitiveness of 
the radiometer and other instruments (130). 
704. For a long time concentrated sulphuric acid was used to moisten the interior 
of the pump, a special stopper being affixed to the pump to admit of the acid being 
run in without interfering- with the exhaustion. For six months observations were 
taken on air and several other gases, when it was found that the presence of sulphuric 
acid lowered the logarithmic decrement, and the work had all to be done over again. 
It is known, from the experiments of Messrs. Wankly^i and Pudbinson, that at a high 
temperature, and under conditions which admit of diffusion taking place, sulphuric 
acid dissociates into sulphuric anhydride and water ; so it is not improbable that at a 
high exhaustion a similar decomposition may take place, one constituent being 
retained in the liquid acid more readily than the other, which escapes into the 
apparatus and alters the logarithmic decrement. 
VISCOSITY OF KEROSOLINE VAPOUR. 
705. The rapid diminution of viscosity in the last experiment after reaching the 
pressure of 400 millims. is probably due to the aqueous vapour in the air being near 
its liquefying point. It was thought advisable to test this hypothesis by employing a 
somewhat less easily condensible vapour, which could be introduced into the apparatus 
without any admixture of air. An experiment was accordingly tried with a very 
volatile hydrocarbon, commercially known as kerosoline, boiling at a little above the 
ordinary temperature. The vapour of this body was introduced into the well- 
exhausted apparatus, when the gauge at once sank 82'5 millims. After the usual 
precautions to eliminate air a series of observations were taken, with the following 
results :— 
Table YIII. — Log. dec. of kerosoline vapour, at pressures between 82 - 5 millims. 
and 8 millims. Temp. 15° C. 
Pressure in 
Log. 
millims. 
decrement. 
82-5 
0-0425 
715 
0-0416 
64-0 
0-0409 
59-0 
0-0407 
5P0 
0-0404 
48-5 
0 0400 
43 0 
0-0396 
38-0 
0-0394 
32-5 
0-0389 
21-0 
0-0381 
17-5 
0-0380 
134 
Ol 
00 
CO 
6 
10-0 
0-0381 
8-0 
0-0379 
3 K 2 
