366 Prof. R. W. Wood on the Condensation 



If the bulb is kept at room temperature no trace of a 

 deposit appears, for reasons which will be given presently ; 

 but if the wall opposite the constriction in the tube is cooled 

 with a small pad of cotton wet with liquid air, a small 

 circular deposit of the metal at once forms, showing that the 

 molecular stream shoots across the vacuum-bulb, without 

 spreading out laterally. If the vacuum is sufficiently high, 

 prolonging the experiment for fifteen or twenty minutes 

 causes no appreciable increase in the diameter of the circular 

 spot, which becomes distinctly visible in ten seconds, after the 

 cadmium has reached the proper temperature (which is only 

 a little above the melting-point). If the vacuum is inferior, 

 the diameter of the spot may be much greater, owing to the 

 deflexion of the cadmium molecules by the residual gas. 



A photograph of the bulb with the circular deposit is 

 reproduced on Plate VII. fig. 1. 



The most instructive way of performing the experiment is 

 as follows. 



The bulb and tube are kept at room temperature until the 

 cadmium melts. If now the side of the bulb is touched at X 

 with a pellet of cotton wet with liquid air, a large deposit of 

 irregular shape at once forms, showing that the bulb is filled 

 with cadmium vapour (with 3-dimensional motion). It 

 seems rather paradoxical that we can have a bulb at room 

 temperature filled with cadmium vapour, which apparently 

 can be condensed only by the application of liquid air. If the 

 bulb is touched by the cotton pellet for only a second or two 

 the deposit forms, and continues to increase in thickness 

 indefinitely, after the removal of the cotton, for the chance 

 of reflexion at room temperature from a cadmium surface is 

 zero, i. e. condensation occurs at the first collision. 



If now the wall of the bulb opposite the constriction is 

 touched with liquid air, the small circular spot at once forms, 

 and goes on building up indefinitely after the removal of the 

 cotton. No further deposit occurs on the side of the bulb, 

 for the circular deposit, once formed, serves as a trap for the 

 one- dimensional molecular stream which impinges on it. 



These phenomena will be more fully discussed later on. 



Tube (b) was designed for obtaining a deposit after reflexion 

 of the molecular stream from a flat polished surface of glass. 

 In order to obtain films thick enough for quantitative mea- 

 surements, it is necessary to keep the reflecting surface at a 

 moderately high temperature for at least 30 minutes. This 

 was accomplished by a small electrical heating-coil introduced 

 into the tube which carried the reflecting surface. The tube 

 was closed in a flame and the closed end ground flat and 



