CONDENSATION AND EVAPORATION 289 



a temperature below about —90° C. If, by cooling the bulb for a moment 

 with liquid air, a deposit is started, this continues to grow in thickness even 

 after it is warmed to room temperature. From these and similar observa- 

 tions, Wood concludes that : 



1. Cadmium atoms all condense on cadmium surfaces at any tem- 

 perature. 



2. Cadmium atoms condense on glass only if it is at a temperature 

 below about —90° C. At higher temperatures, nearly all the atoms 

 are reflected. 



This viewpoint leads to no explanation of the changes in the reflection 

 coefficient. The results of Wood's experiments may, however, be explained 

 by the theory that all the atoms, striking either the glass or the cadmium 

 surface, condense, and that subsequent evaporation accounts for the ap- 

 parent reflection. 



Cadmium atoms on a glass surface are acted on by totally different forces 

 from those holding cadmium atoms on a cadmium surface. When a thick 

 deposit of cadmium which has been distilled onto glass in vacuum, is 

 heated quickly above its melting-point, the molten cadmium gathers to- 

 gether into little drops on the surface of the glass. In other words, molten 

 cadmium does not wet glass. Therefore cadmium atoms have a greater 

 attractive force for each other than they have for glass. Thus, single 

 cadmium atoms on a glass surface evaporate off at a lower temperature 

 than that at which they evaporate from a cadmium surface. It is not un- 

 reasonable to assume that in Wood's experiments, even at —90° C, the 

 cadmium evaporated off of the glass as fast as it condensed upon it. 



This theory possesses the advantage that it automatically explains the 

 apparent reflection of cadmium atoms from a glass surface at room tem- 

 perature, and indicates why this effect should be absent at low temperatures. 



We shall see, moreover, that this condensation-evaporation theory ex- 

 plains many other facts incompatible with the reflection theory. 



Let us examine for a moment the essential differences between these 

 two theories. Wood describes his remarkable experimental results, but he 

 has not attempted to discuss the mechanism of the underlying processes. 

 It is clear that Wood uses the term 'reflection' merely to express the fact 

 that under certain conditions no visible deposit is formed when the atoms 

 strike a surface. From this point of view, condensation followed by 

 evaporation is the same as reflection. In considering the possible mecha- 

 nisms of the process, however, we must sharply distinguish between the two 

 theories. 



When an atom strikes a surface and rebounds elastically from it, we 

 are justified in speaking of this process as a reflection. Even if the collision 

 is only partially elastic, we may still use this term. The idea that should 



