300 
:>rR. c. T. R. wiLsox ox COXDEXSATION OF v:ater vapour 
If we consider tlie experimental results obtained with air on March 3rd, we have, 
when the violet colour results, Vo/i'i = 1’420, the initial temperature being 19° C.. 
and the pressure, when the volume = and the temperature = 19° C., being equal to 
1039 milliras. of mercury. The density of air at standard temperature and pressure 
is '00129 gi'in. per cubic centimetre. 
Therefore, 
M — -00129 X X 
7b0 
'001 65. 
Tlie lowest temperature calculated from the expansion is 
Also. 
U = - 20'2. 
Pi - do X 
where — density of saturated steam at the temperature b- 
When b = 19° C. 
Pq = -0000162, 
therefore, 
p^ = -0000162 X = -000114. 
Now G, the specific heat of air at constant volume = ‘167. 
We, therefore, find for the density of the vapour, wdren the temperature has risen 
from b to b 
p = -000014 — 
•167 X '00165 {t — t. 2 ) 
606 
= -000014 - 4-55 X 10-7 {t - u). 
If we put t = — 4° C., we obtain for p the value 4-0 X lO-*'. Now, the density of 
the saturated vapour at that temperature is 3’7 X 10-*". More vapour would, there¬ 
fore, condense, and the temperature would rise further. 
If i = — 3° C., p calculated from the above equation = S'O X 10-7; but the density 
of saturated vapour at — 3° G. is 4-0 X 10-7. Gondensation will, therefore, not go 
so far as this, but only till the temperature rises to about — 3‘5° G., and the density 
of the vapour has fallen to about 3'8 X 10-*' grm. per cubic centimetre. 
This gives us for the quantity of -vvater which separates out from each cubic centi¬ 
metre 
- p.= 11-4 X 10-^^ - 3-8 X 10-'^ 
= 7-6 X lO-'^ gvm. 
In considering how far the condensation would go, the density of vapour in equi¬ 
librium when in contact with drops of the size of those actually present should have 
