and Reflexion of Gas Molecules. 369 



Experiments were next made to determine the temperature 

 necessary for the formation of a homogeneous film of cadmium. 



Fiff. 2. 



The bulb was cooled by the application of alcohol a little 

 above its solidifying point (with a consistency of Canada 

 balsam), but no trace of a film could be detected at the end 

 of five minutes. Solid alcohol, however, caused a film to 

 appear in ten or fifteen seconds. As the alcohol contained 

 some water, its freezing-point was in the neighbourhood of 

 — 100°, which can be considered as the approximate critical 

 temperature for cadmium. Mercury gave no film in a bulb 

 cooled to —130°, but a film immediately appeared if the 

 temperature was reduced to —150°. The critical tempe- 

 rature for mercury is therefore in the neighbourhood 

 of -140°. 



I am not yet certain how sharply these temperatures can 

 be determined, or what the nature of the deposit will be for 

 prolonged action at a temperature a few degrees above 

 the critical temperature ; but it seems probable that in this 

 case a granular deposit will be formed made up of exceedingly 

 small grains very close together. 



As I have said before, if any portion of the inner surface 

 of the bulb has received a low-temperature film deposit, a 

 similar type of condensation continues even at room tempe- 

 rature, the rest of the bulb remaining clean. 



These experiments show pretty clearly how condensation 

 takes place on metal and glass surfaces. In the case of 

 surfaces composed of the same metal as the vapour, the 

 molecules stick at the first impact, the probability of reflexion 

 being zero, even at room temperature. The same is true 

 for a surface, such as glass, if it is cooled below a certain 

 critical temperature which is characteristic for each sub- 

 stance investigated. If the temperature is above this, the 

 probability of reflexion is large, and only a few molecules 



Phil. Mag. S. 6. Vol. 32. No. 190. Oct. 1916. 2 C 



