750 Messrs. Owen and Hughes on Molecular 
Comparing tins table with Table I. it will be noticed that 
the " nucleating temperature " for the spiral tester X of 3 c.c. 
and for the testers Y and Z are respectively -131°, —97°, 
— 80°. It will be noticed that there is a considerable difference 
between the nucleating temperatures obtained with testers X 
and Y while there is a much smaller difference between the 
values obtained for testers Y and Z, although in the latter case 
the ratio of volumes is 6:1 whereas in the former only 2:1. 
Xow the internal diameter of the tube forming tester Y was 
7 mm. while that of the tube forming the spiral tester X was 
only 3 mm. The rush of the gas into X is therefore not 
nearly so sudden as into the wider tube Y. On the other 
hand since Y and Z are both wide the difference in the rate 
at which the gas rushes into them will not be so marked. 
The above variations are therefore due, we think, not so 
much to the differences in the volumes of gas cooled as to 
variations in suddenness of cooling due to differences in 
the size and shape of: the testers. 
The different effects obtained by the " slow " and " sudden " 
methods of cooling, as illustrated by Table I., naturally 
suggested trying a very slow rate of cooling. We used the 
following method to cool the gas very slowly right down to 
the temperature of liquid air. 
The tester X with tap M open (see fig. 2) was enclosed 
in a stout brass box, but not actually in contact with it. 
Through the lid of the box passed a thermometer, and a 
propeller for stirring up the air inside. By surrounding 
the box with liquid air the temperature fell very slowly 
until it became almost stationary at —152°. The box 
was then quickly removed and replaced by a vessel of liquid 
air. The whole process took about twenty minutes. The 
result obtained was a few drops, practically a no effect, whereas 
by the ordinary " slow " method a rain cloud was obtained 
(see Table I.). In all probability the few drops actually 
obtained were due to the more rapid fall of temperature from 
-152° to -190°. 
II. The Effect in PupvE Gases. 
Carbon Dioxide. 
In the first paper we mentioned that the effect is much 
more marked in carbon dioxide when some of the gas is 
actually solidified during the cooling process, and suggested 
that the nuclei are produced, not while the gas is approaching 
the solid state, but while subliming from the solid back 
