290 
.MR. C. T. R. WILSON ON CONDENSATION OF WATER VAPOUR 
pressure as possible. Hydrogen was thus generated and pumped out several times, 
and lastly a vigorous stream of hydrogen was allowed to escape through the water in 
the flask F (fig. 2 ) by opening the taps T 3 and T^. The flame was then removed and 
T 3 closed, the gas which still continued to come ofif escaping through the safety- 
tube D. The hydrogen was then ready to be pumped into the expansion apparatus 
as required. Even in the cold, hydrogen is slowly formed by the water vapour 
reaching the sodium by diffusion, and escapes through the safety-tube. No further 
heating was therefore required when a fresh charge was wanted in the expansion 
apparatus. 
Results.—-The phenomena attending condensation in presence of hydrogen differed 
considerably from those observed with other gases. 
As in air and oxygen, dense condensation begins when is between 1‘37 and 
1'38, and the number of particles rapidly increases with increasing expansion. With 
very slightly smaller expansions, however, the drops are excessively few, and if 
be less than 1'36, they are either absent altogether or at the most one or two 
Fig. :i. 
scattered drops are seen. It was found impossible to get any consistent measure¬ 
ments of the minimum expansion required to make these drops appear. In no case 
was any condensation at all observed when vjvi was less than 1'30, while in one 
series of observations no condensation resulted when was as great as 1’356. 
It is likely that this irregular condensation is due to impurities in the gas. For 
the observations in which no condensation whatever was observed, even when tq/yj 
was not much below 1'36, were all made when the hydrogen was comparatively fresh, 
before it had been allowed to expand more than a very few times. The slight 
contamination which may take place by diffusion of air through the water lubricating 
the piston may account for their subsequent appearance. The contamination which 
can take place in this way must, as has been shown, be very slight; but it is quite 
