chapter Fourteen 



THE CONDENSATION AND EVAPORATION 



OF GAS MOLECULES 



Several years ago, ( i ) I gave evidence that atoms of tungsten, molyb- 

 denum, or platinum vapors, striking a clean, dry glass surface in high 

 vacuum, are condensed as solids at the first collision with the surface. 

 Subsequently, similar evidence (2) was obtained in connection with a study 

 of chemical reactions in gases at low pressures. It was concluded that in 

 general, when gas molecules strike a surface, the majority of them "do 

 not rebound from the surface by elastic collisions, but are held by cohesive 

 forces until they evaporate from the surface." In this way a theory of 

 adsorption was developed (3) which has been thoroughly confirmed by 

 later experiments. It was stated : "The amount of material adsorbed de- 

 pends on a kinetic equilibrium between the rate of condensation and the 

 rate of evaporation from the surface. Practically every molecule striking 

 the surface condenses (independently of the temperature). The rate of 

 evaporation depends on the temperature (van't Hoff's equation) and is 

 proportional to the fraction of the surface covered by the adsorbed 

 material." 



R. W. Wood (4) described some remarkable experiments in which a 

 stream of mercury atoms impinges upon a plate of glass held at a definite 

 temperature. With the plate cooled by liquid air, all the mercury atoms 

 condense on the plate, but at room temperature all the atoms appear to be 

 diffusely reflected. 



The whole question of the evaporation, condensation, and possible re- 

 flection of gas molecules has been discussed at some length in two recent 

 papers (5), (6). It was pointed out that, in Wood's experiments, there are 

 excellent reasons for believing that the mercury vapor actually condenses 

 on the glass at room temperature, but evaporates so rapidly that no visible 

 deposit of mercury is formed. Further evidence of the absence of reflection 

 is furnished by the operation of the 'Condensation Pump' (7). 



In a second paper. Wood (8) gives an account of some still more strik- 

 ing experiments. A stream of cadmium atoms, striking the walls of a well 

 exhausted glass bulb, does not form a visible deposit unless the glass is at 



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