PHYSICS: I. LANGMUIR 
145 
no foggy deposit is formed, although when the same quantity of cad- 
mium is made to impinge against the surface in a shorter time (and 
therefore at higher pressure) a foggy deposit results. This fact con- 
stitutes 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 previously cooled by liquid air. 
The question arises: how much cadmium could have condensed on 
the bulb in one minute while the lower part of the bulb was at 60°C.? 
The vapor pressure of cadmium has been determined by Barus^ 
between the temperatures 549° and 770°C. If the logarithms of the 
pressures are plotted against the reciprocals of the temperature, a 
straight line is obtained from which the following equation for the vapor 
pressure (in bars) is obtained as a function of absolute temperature 
6060 
log^ = 11.77 - — (1) 
At 60° C. the vapor pressure of cadmium is of the order of magnitude 
of 4 X 10~^ bars. Now the number of molecules of gas which strike 
a square centimeter of surface per second is 
^ = 2.65 X lO^'p/y/Wr (2) 
Substituting M = 112, T = 333°, and = 4 X 10-7, we find that 
with saturated cadmium vapor at 60°C., ^ = 5 X 10^^ atoms per sec- 
ond per square centimeter. 
The maximum number of atoms of cadmium which can condense in 
one minute on a spot cooled in liquid air when the lower part of the bulb 
is at 60°C. is therefore 3.0 X 10^^ atoms per square centimeter. The 
diameter of a cadmium atom is approximately 3. 1 X 10""^ cm., so that 
it would require 1 . 0 X 10^^ atoms to cover 1 square centimeter with a 
single layer of atoms. 
Therefore the deposit which forms in one minute with the vapor from 
cadmium at 60°, contains only enough cadmium atoms to cover 3/1000 
of the surface of the glass. Yet this deposit serves as an effective nu- 
cleus for the formation of a visible deposit. 
If the lower part of the bulb is heated to 78° instead of 60°, the 
