144 
PHYSICS: I. LANGMUIR 
uniform in density. It is thus evident that traces of residual gas may 
prevent the growth of the deposit, particularly in those places which 
have been the most effectively cooled. This is probably due to the ad-, 
sorption of the gas by the cooled metal deposit. This gas is appar- 
ently retained by the metal, even after it has warmed up to room tem- 
perature, so that vapor condensing on the surface evaporates off again 
at room temperature. 
These results indicate how enormously sensitive such metal films are 
to the presence of gas. However, by using liquid air and charcoal con- 
tinually during the experiments, most of these complicating factors 
were eliminated. 
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 fifteen seconds. This deposit is very different from that ob- 
tained by cooling the bulb in liquid air. Microscopic examination 
shows that it consists of myriads of small crystals. According to the 
condensation-evaporation theory, the formation of this fog is readily 
understood. Each atom of cadmium, striking the glass at room tem- 
perature, remains on the surface for a certain length of time before evap- 
orating 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 evapo- 
ration 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 propor- 
tional 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 condensing 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, 
