142 
PHYSICS: I. LANGMUIR 
that no visible deposit of mercury is formed. Further evidence of the 
absence of reflection is furnished by the operation of the 'Condensa- 
tion Pump.'^ 
In a second paper, Wood^ gives an account of some still more striking 
experiments. A stream of cadmium atoms, striking the walls of a well 
exhausted glass bulb, does not form a visible deposit unless the glass is 
at 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 observations. Wood concludes that: 
1. Cadmium atoms all condense on cadmium surfaces at any 
temperature. 
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 reflec- 
tion 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 ac- 
counts for the apparent 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 together 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 sur- 
face. It is not unreasonable 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 
temperature, and indicates why this effect should be absent at low tem- 
peratures. 
We shall see, moreover, that this condensation-evaporation theory 
explains 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 
