PHYSICS: I. LANGMUIR 
143 
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, con- 
densation followed by evaporation is the same as reflection. In consid- 
ering the possible mechanisms of the process, however, we must sharply 
distinguish between the two theories. 
When an atom strikes a surface and lebounds elastically from it, we 
are justified in speaking of this process as a reflection. Even if the col- 
lision is only partially elastic, we may still use this term. The idea that 
should be expressed in the word 'reflection' is that the atom leaves the 
surface by a process which is the direct result of the collision of the 
atom against the surface. 
On the other hand, according to the condensation-evaporation theory, 
there is no direct connection between the condensation and subsequent 
evaporation. The chance that a given atom on a surface will evaporate 
in a given time is not dependent on the length of time that has elapsed 
since the condensation of that atom. Atoms striking a surface have a 
certain average 'life' on the surface, depending on the temperature of 
the surface and the intensity of the forces holding the atom. Accord- 
ing to the 'reflection' theory, the life of an atom on the surface is sim- 
ply the duration of a collision, a time practically independent of tem- 
perature and of the magnitude of the surface forces. 
To determine definitely which of the two theories corresponds best 
with the facts, I have repeated Wood's experiments under somewhat 
modified conditions. A small spherical bulb, together with an ap- 
pendix containing cocoanut charcoal, was heated to 600°C. for about 
four hours while being exhausted by a condensation pump. A liquid- 
air trap was placed between the pump and the bulb. Some cadmium 
was purified by distillation and was distilled into the bulb, which was 
then sealed off from the pump. The cadmium in the bulb was then all 
distilled into the lower hemisphere. By heating this lower half of the 
bulb to about 140°C., the upper half remained clear, but by applying 
a wad of cotton, wet with liquid air, to a portion of the upper hemi- 
sphere, a uniform deposit formed within less than a minute and con- 
tinued to grow, even after the liquid air was removed. 
When the liquid air was applied only long enough to start a deposit, 
it was found in the first experiments that the deposit did not grow 
uniformly, but became mottled, or showed concentric rings. The 
outer edges of the deposit were usually much darker than the central 
portions. By cooling the cocoanut charcoal in liquid air, this effect 
disappeared entirely and the cadmium deposits became remarkably 
