Nuclei produced in Gases by Sudden Cooling. 741 
exhausted through D' to a low pressure by means of a water- 
pump and is then surrounded by the cold liquid. When the 
Fig.L 
Fig. 2. 
tester has attained the temperature of the liquid, the tap M 
is quickly turned and the gas rushes from B into it while the 
mercury is run up to bring the pressure to the desired value. 
After some thirty seconds, during which the temperature 
has risen about one degree, the cooling liquid is removed. 
The gas is then given a minute and a half to regain its 
normal temperature ; at the end of which it is driven into 
the cloud-chamber Q where it is tested for the presence of 
nuclei. This method of cooling we shall denote as the 
" sudden" method, while the original method may be called 
relatively the u slow " method. The " sudden " method of 
cooling possesses one great advantage over the " slow " 
method, viz., that it enables the temperature to which the 
gas is cooled to be determined much more accurately ; for 
when the tester is surrounded by the cold liquid which is 
kept vigorously stirred by a small rotating screw, the 
temperature of the liquid rapidly rises six or seven degrees, 
and then remains nearly constant, rising only a degree or so 
per minute. With the " slow " method of cooling the mean 
of the initial and final temperatures (which differed by 
several degrees) has to be taken as the temperature to which 
the gas is cooled, an assumption which is perhaps hardly 
justifiable. But with the " sudden " method the gas is not 
admitted into the cooled evacuated tester until the temperature 
of the latter has become practically constant. Thus, by this 
