296 
C. T. R. WILSON ON CONDENSATION OF WATER VAPOUR 
density calculated on the assumption that the vapour behaves like a perfect gas. We 
may, therefore, write 
_ !0 w ^2 
P2 '^2 
Therefore, S 
Supersaturation Bequired for Rain-like Condensation. 
For convenience of comparison the calculations have been made for the case where 
the initial temperature is 20° C. As we have seen, the ratio when conden¬ 
sation just begins, is within the limits of experimental error, constant within the 
ordinary range of room temperature. 
Chlorine is too far removed from the condition of a perfect gas for the fall of 
temperature to be calculated. The error from the sa,me cause may also be consider¬ 
able in the case of COo. 
Ill the following table Regnault’s numbers have been used for the vapour pressure 
at temperatures above the freezing point. The vapour pressui’es over water below the 
freezing point are from a paper by Juhlin,* who has measured directly the pressure 
over water over a considerable range of temperature below the freezing point, t-^, t .2 
are the initial and final temperatures Centigrade, Oi = 273, ^2 = C + 273. 
A- 
_ 
"]• 
r,!ry 
7- 
OJO.,. 
/o. 
Air. 
293 
17-39 
1-2.52 
1-41 
1-097 
- 5-8 
3-02 
4-2 
Oxygeu .... 
293 
17-39 
1-257 
1-41 
1-098 
- 6-2 
2-93 
4*0 
Nitrogen .... 
293 
17-39 
1-262 
1-41 
1-100 
- 6-7 
2-83 
4-4 
Carbonic Acid . 
293 
17-39 
1-365 
1-31 
1-101 
- 6-9 
2-78 
4-2 
(Chlorine 
293 
17-39 
1-30 
1-32 
1 087 
— 3*5 
3-58 
3-4) 
. 
Hydrogen does not appear in this table, as no regular rain-like condensation was 
observed. 
It will be noticed that the supersaturation required to cause condensation is 
practically the same in COo as in the other gases, in spite of the great difference 
in 
The supersaturation is in each case the greatest which can exist at the temperature 
C. without condensation resulting. 
To find to what extent this depends upon the temperature, w^e may make use of 
tlie fact that ro/i'i, when the resulting supersaturation is just sufficient to cause 
condensation, is in the case of air constant for temperatures between 15'5°C. 
and 28-8° C. 
* ‘ JRbaiig till K S. Yet. Akad. Haiidliugar,’ Bd. 17. 
