GASES BY THE ACTION OF RONTGEN RAYS AND OTHER AGENTS. 
413 
The effects described below are certainly due to the ultra-violet rays. That they 
are due to light of some kind is easily shown by interposing a quartz lens, so that an 
image of the source is formed, which, by a slight displacement of the lens, may be 
made to fall either on the quartz window or just to one side of it. In the latter case 
all effect on the condensation ceases, while so long as the concentrated light from the 
quartz lens does enter the window, the effect is immensely increased by its presence. 
Even exceedingly thin glass or mica interposed anywhere between the source and the 
cloud-chamber prevents all action. It is, therefore, the ultra-violet rays alone which 
are active in producing nuclei. 
The results of one series of experiments, extending over four consecutive days, are 
given in the tables which follow. Many experiments of the same kind were made 
with exactly similar results. Since, however, as will be seen in what follows, the 
absolute numbers are of no particular interest, the one series has been considered 
sufficient. For the same reason the value of rff ci has not been calculated, the gauge 
reading jy (approximately equal to Pj — Pg) being used as a measure of the amount 
of expansion. The time for which the air was exposed to the ultra-violet rays before 
the expansion was made is given, as well as the interval, if any, which elapsed 
between the cutting-off of the rays and the expansion. When the number under this 
last heading is zero, it is to be taken as indicating that the expansion was brought 
about while the sparks were still passing. 
Distance of Spark from Quartz Plate = 4'5 centims. 
p = gauge reading 
(approx. = pressure fall). 
Duration of 
exposure. 
Interval after 
exposure. 
Result of expansion. 
(1) 10 millims. 
1 min. 
0 
Fog 
(2) 112 „ 
10 mins. 
5 mins. 
Dense coloured fog 
Distance of Spark from Quartz Plate =17 centims. 
p = gauge reading 
(approx. = pressure fall). 
Duration of 
exposiu'e. 
Interval after 
exposure. 
Result of expansion. 
(.3) 102 millims. 
5 mins. 
0 
0 
(4) 1.33 „ 
30 secs. 
0 
Slight shower 
(5) 133 „ 
7 mins. 
0 
Slight shower 
(6) 14.5 „ 
30 secs. 
0 
Shower 
(7) 145 „ 
5 mins. 
0 
Shower 
(8) 160 „ 
30 secs. 
0 
Fog 
(9) 157 „ 
10 „ 
0 
Very dense shower 
