produced by Cooling Gases to Low Temperatures. 531 



the end of this interval the mercury was again brought to 

 the fixed mark P (so that the gas was once more at a pressure 

 of 80 centims.). The tap D was now opened and the mercury 

 quickly raised to a second fixed mark near the top of the tube, 

 thereby driving into the cloud chamber practically the whole 

 of the gas which had undergone the cooling. D was then 

 closed and an expansion of about 1*10 made, completing the 

 test. Between every test, whether nuclei had been produced 

 or not, a stream of fresh gas was drawn through the apparatus 

 for several minutes. At frequent intervals control tests were 

 performed identical with that described above omitting 

 the cooling process only. Reasons are given below for 

 selecting the stated duration of the cooling operation, the 

 time allowed for the gas to warm up, and the value of the 

 expansion used. It remains to add that always whenever 

 the apparatus required resetting-up, it was first washed with 

 caustic potash, nitric acid, and, after rinsing several times 

 with clean water, thoroughly dried. 



General Results. 



The following facts in regard to the nuclei are general in 

 that they apply to all the gases in which the nuclei were 

 obtained. 



(1) After passing below the " critical temperature " * the 

 number of nuclei produced increases rapidly with the degree 

 of cooling, the maximum effect being obtained when the 

 cooling is sufficient to cause some of the gas to turn (or to 

 be on the point of turning) into the liquid state. At this 

 stage the number of nuclei produced is generally large enough 

 to cause coloured clouds. 



(2) The number of nuclei produced is independent of the 

 time of cooling provided the time is long enough to allow the 

 gas in the tester to fall to the temperature of the cooling 

 liquid. This time was about 25 seconds with liquid air as 

 the cooling agent. We allowed, however, a little margin, 

 and in all the experiments described below the time of cooling 

 was 40 seconds. 



(3) The nuclei show a remarkable persistency. If an 

 interval of 10 minutes be allowed to elapse between the 

 removal of the cooling mixture and the passage of the gas 

 into the cloud chamber, the shower obtained is almost as 

 dense as it is when the warming-up process takes only one 



* The term " critical temperature " in this paper means always that 

 temperature to which a gas has to be cooled before the production of 

 nuclei can be detected. 



