758 
Messrs. Owen and Hughes on Molecular 
atmospheric temperature, and immediately the tap K was 
closed, the " oven " was surrounded by hot glycerine for a 
definite length o£ time. A similar experiment was then made 
in which the " oven " was kept at atmospheric temperature 
for the same time. Before admitting into the cloud-chamber, 
the gas in the " oven " was allowed one minute in which to 
regain atmospheric temperature. 
Table VII. compares the persistency of air nuclei at 
different temperatures. 
Table VII. 
Time and Tempe- 
rature in " Oven." 
-pm , | Time and Tempe-i -,-,#> , 
Effect. >■ , ■ ur\ »\ Effect. 
j rature in Oven. 
2 min. at 15° 
4 min. at 15° 
9 min. at 15° 
Keavv shower ' l min ' at 100 °- 1 
Heavy snower. ,, j min# at ^ j 
ri • j i 3 min. at 100°. 1 
Good shower. x min> afc ^j 
-c -, i 8 min. at 100°. 
iew drops. ■ -. . , n KO 
1 1 mm. at 15°. 
Fair shower. 
Yery thin 
shower. 

Time and Tempe- 
rature in " Oven." 
Effect. 
1 min. at 185°' "1 
1 min. at 15°. j 
2 min. at 185°. "1 
2 min. at 15°. J 
Fair shower. 

The above table shows clearly that keeping the nuclei at 
a high temperature aids their disappearance. This suggested 
that the nuclei would be still more persistent if kept at a low 
temperature. This was verified, for it was found that after 
thirty minutes at —75° a fair shower was obtained which is 
greater than the effect obtained after a much shorter period 
at 15° (see Table VL). 
Thus raised the interesting question as to whether the 
nuclei would persist indefinitely if kept at a temperature at 
which they were produced. To investigate this point an 
experiment was carried out with tester Z (see fig. 3). When 
the nuclei were produced by thfi " sudden " method at the 
temperature of liquid air, the effect obtained a minute and a 
half later was a thick white fog. On producing the nuclei 
in the same way, but keeping them for two hours in the tester 
