AND CHLOEINE UNDEK THE INFLUENCE OF LIGHT. 
105 
the “dark” and “light” expansions is not to be attributed merely to the difficulty 
of observing the cloud in the former. 
(I.) Experiments with Chlorine alone. — C. T. R. Wilson showed that in dust-free 
chlorine a slight rain-like condensation took place, when an expansion, measured 
by Pj/Pg, greater than TSO and less than 1-45, was made. Pj is the pressure 
before expansion, P 3 after expansion. With P 1 /P 3 greater than 1 - 45 , a cloud-like 
condensation occurs, increasing in thickness as the amount of expaiision increases. In 
the present experiments the cloud-like condensation was observed to begin at 
P 1 /P 3 = 1’50 approximately, and up to this value there were only a few scattered 
drops observed on expansion, when the chlorine was darkened. If, liowever, the 
chlorine be strongly illuminated, the phenomena are very different. The number of 
drops increases as the expansion increases from 1-27 up to the value 1-46, 
when a thick fog is the result of the expansion. The following table represents the 
observations in a series of experiments of this kind :— 
P 1 /P 2 . 
Observation. 
1-21 
No drops seen. 
1-24 
1-27 
A few drops. 
1-30 
Fine rain. 
1-33 
1-36 
More rain with finer drojrs. 
1-41 
Thick rain. 
1-43 
Rain cloud. 
1-46 
Thick cloud. 
1 
If the chlorine be darkened, after a few expansions to precipitate nuclei left by the 
evaporation of the cloud, the behaviour is similar to that of the original chlorine ; 
only a few drops are seen until the expansion reaches the limit Pj/Pg = 1'5. 
These experiments were repeated several times with light of varying intensity. 
The same result appeared always, so that it is clear that ordinary light (not ultra¬ 
violet light) produces in damp chlorine some substance which can act as nuclei for 
condensation. 
( 2 .) Experiments with the Mixture of Hydrogen and Chlorine. —In the dark, 
expansions with Pj/Pj less than 1’42, only a few drops were observed. With the 
expansion gieater than this amount, the fog-like supersaturation cloud aiipeared. 
With the mixture illuminated, the results were similar to those obtained with 
chlorine alone. The formation of drops began when the expansion was such that 
= 1 ' 22 , the number increasing with increasing expansion, until a thick cloud 
was produced with P 1 /P 2 = 1’36. If the intensity of the light was altered, the cloud’s 
thickness altered, but the same variation between the limits, 1 ‘ 22 , 1 ' 86 , occurred. 
The next experiments made were to determine the effect of the time of illumination 
VOL. COIL—A. 
P 
