AND CHLORINE UNDER THE INFIUTENCE OF IJGHT. 
107 
chlorine alone suggests that in the mixed gases it is the chlorine alone that is 
efficient in producing the nuclei, and that the hydrogen is inactive. It might be 
expected that the cloud in the mixture might be formed on hydrochloric acid 
molecules produced after the illumination. But the cloud-forming nuclei appears 
before any hydrochloric acid is formed in measurable quantity, as the following 
experiments show. The expansion apparatus was joined to the ordinary insolation 
apparatus of fig. 2, and the two illuminated with the same light. The time to 
the first appearance of motion of the water index, indicating the absorption of 
hydrochloric acid, was noted. Expansions were also made and the character of the 
cloud after different times of illumination observed. In all cases the cloud produced 
on expansion had reached its densest appearance a considerable time before the 
hydrochloric acid began to be foi’med. The following numbers will show the kind of 
phenomena observed 
Time to the first motion of water index of insolation apparatus .... 29 seconds. 
Time of insolation 
before expansion in 
expansion vessel 
Pi/P.2 = 1-38. 
Observation. 
1 second 
Rain. 
2 seconds 
Rain cloud. 
•5 
Thick cloud. 
and no change in the character of the cloud appeared with longer exposures. 
For more careful experiments of tliis kind a long capillary tube was sealed to the 
exit tube of the expansion bulb, in which a drop oT water served as an index, so that 
the expansion bulb itself could be used as the insolation bulb (fig. 17). If the tap 
between the capillary tube and the expansion bulb was open, the drop of water in 
the index tube defined the volume of the gas in the expansion bulb; and the rate of 
formation of hydrochloric acid in the bulb could be followed by noting the motion of 
the index drop. With the same tap closed an expansion could be made of the same 
p 2 
