42 W. Thompson on the size of Atoms. 
the quantity required to raise the temperature of the liquid 
by one degree centigrade. This is far more than we can admit 
as a possible amount of work done in the extension of a liquid 
film. smaller amount of work spent on the liquid would 
convert it into vapor at ordinary atmospheric pressure. The con- 
clusion is unavoidable, that a water-film falls off greatly in its 
contractile force before it is reduced to a thickness of a twenty- 
millionth of a millimeter. It is scarcely possible, upon any 
conceivable molecular theory, that there can be any considera- 
ble falling off in the contractile force as long as there are 
several molecules in the thickness. It is therefore probable 
that there are not several molecules in a thickness of a twenty- 
millionth of a millimeter of water. 
The kinetic theory of gases suggested a hunded years ago 
by Daniel Bernouilli has, during the last quarter of a century, 
been worked out by Herapath, Joule, Clausius, and Maxwell, 
to so great perfection that we now find in it satisfactory ex- 
lanations of all non-chemical properties of gases. However 
lineal paths and constant velocities by collisions or mutual 
influences, so rare that the mean len a proximately recti- 
lineal portions of the path of each molecule is many times 
greater than the average distance from the center of each mole- 
cule to the center of the molecule nearest it at any time. If, : 
for a moment, we suppose the molecules to be hard elastic 
eq t 
ae of free path. But we cannot believe that the individual 
molecules of gases in general, or even of any one gas, are 
vary, —— variations of the distance, so as to fulfil some 
definite law. 
~ 
