284 
MR. C. T. R. WILSON" ON CONDENSATION OF WATER VAPOUR 
certain conditions. They have been investigated by R. V. Helmholtz/”' Aitken"^ 
and, in a more elaborate way, by BARUS.j 
Measurements of the Ex]jansion required to produce Cloud-like 
Condensation. 
The transition from rain-like to cloud-like condensation is sudden enough to 
enable one to measure, with considerable accuracy, the value of the ratio vfv-^ when 
cloud-like condensation just begins. There is, in fact, a second condensation point, 
below which the drops are few, and the number shows only a slight increase with 
increasing expansion ; while above it the number increases at an excessively rapid 
rate with increasing expansion. 
Expansion required for Cloud-like Condensation in Saturated Air. 
Date. 
B. 
t°C. 
TT. 
Pi- 
Pi- 
P2- 
Pj- 
P1/P3 = 
Result. 
February 14. 
775 
17-5 
15 
706 
1465 
308 
1067 
1-373 
Rain 
„ 14. . 
775 
17-5 
15 
712 
1471 
308 
1067 
1-378 
Fog 
„ 15. . 
775 
18-5 
16 
;9i 
1549 
369 
1127 
1-375 
Rain 
5 , 10, 
775 
18-5 
16 
795 
1553 
369 
1127 
1-378 
Fog 
„ 18. . 
769 
15'0 
13 
803 
1558 
381 
1136 
1-372 
Rain 
„ 18. . 
769 
15-0 
13 
813 
1568 
381 
1136 
1-380 
Fog 
The transition from rain to fog takes place when vfv^ is between 1’37 and 1’38. 
The above results were obtained using the same method of illumination as in the 
experiments on rain-like condensation. Observations were also made with the 
condensing lens removed. 
First Appearance of Diffraction Rings. 
D.ite. 
B. 
C. 
< 
Pi- 
Pn 
P 2 - 
P2- 
P|/P.2 = lyvi. 
February 24 .... 
776 
15 
13 
676 
1438 
279 
1041 
1-381 
25 .... 
772 
13 
11 
667 
1427 
277 
1037 
1-376 
jMarch 2. 
750 
18 
15 
807 
1541 
380 
1114 
1-383 
„ 2. 
750 
18-5 
16 
715 
1448 
318 
1051 
1-378 
IMeaii . 
1-379 
* ‘Wied. Ann.,’ 32, p. I, 1887. 
t ‘ Proc. Roy. Soc.,’ vol. 51, p. 422, 1892. 
t Earus, loc. cit., also ‘ Phil. Mag.,’ vol 35, p. 315, 1893. 
