CONDENSATION AND EVAPORATION 291 



If all the cadmium is distilled to the lower half of the bulb and this is 

 then heated to 220° in an oil bath while the upper half is at room tem- 

 perature, a fog-like deposit is formed on the upper part of the bulb in about 

 hfteen seconds. This deposit is very different from that obtained by cooling 

 the bulb in liquid air. Microscopic examination shows that it consists of 

 myriads of small crystals. According to tlie condensation-evaporation 

 theory, the formation of this fog is readily understood. Each atom of 

 cadmium, striking the glass at room temperature, remains on the surface 

 for a certain length of time before evaporating off. If the pressure is very 

 low, the chance is small that another atom will be deposited, adjacent to the 

 first, before this has had time to evaporate. But at higher pressures this 

 frequently happens. Now if two atoms are placed side by side on a surface 

 of glass, a larger amount of work must be done to evaporate one of these 

 atoms than if the atoms were not in contact. Not only does the attractive 

 force between the cadmium atom and the glass have to be overcome, but 

 also that between the two cadmium atoms. Therefore the rate of evapora- 

 tion of atoms from pairs will be much less than that of single atoms. 

 Groups of three and four atoms will be still more stable. Groups of two, 

 three, four, etc., atoms will thus serve as nuclei on which crystals can grow. 

 The tendency to form groups of two atoms increases with the square of 

 the pressure, while groups of three form at a rate proportional to the cube 

 of the pressure. Therefore the tendency for a foggy deposit to be formed 

 increases rapidly as the pressure is raised or the temperature of the con- 

 densing surface is lowered. 



On the other hand, according to the reflection theory, there seems to be 

 no satisfactory way of explaining why the foggy deposit should form 

 under these conditions. 



Experiments show clearly that when a beam of cadmium vapor at very 

 low pressure strikes a given glass surface at room temperature, no foggy 

 deposit is formed, although when the same quantity of cadmium is made 

 to impinge against the surface in a shorter time (and therefore at higher 

 pressure) a foggy deposit results. This fact constitutes strong proof of the 

 condensation evaporation theory. 



A deposit of cadmium of extraordinary small thickness will serve as 

 a nucleus for the condensation of more cadmium at room temperature. 

 Let all the cadmium be distilled to the lower half of the bulb. Now heat 

 the lower half to 60° C. Apply a wad of cotton, wet with liquid air, to a 

 portion of the upper half for one minute, and then allow the bulb to warm 

 up to room temperature. Now heat the lower half of the bulb to 170° C. 

 In about thirty seconds a deposit of cadmium appears which rapidly grows 

 to a silver-like mirror. This deposit only occurs where the bulb was pre- 

 viously cooled by liquid air. 



