694. Dr. G. Johnstone Stoney on 
frequently made use of in those days was that of a froth of 
bubbles pressing against one another. This erroneous theory 
held the field in Avogadro’s time and for more than thirty 
years afterwards; but in the fifties of the nineteenth century 
it was gradually, though not without protest, displaced (chiefly 
through a masterly series of papers by Clausias) by the 
Kinetic Theory, which is now the prevalent theory. The 
Kinetic Theory of Gas, as formulated by Clausius, regards the 
molecules of a gas as missiles of equal mass, darting about in 
space and not acting sensibly on one another except when 
‘‘encounters’’ chance to take place, 7. ¢., not until the 
centres of mass of two molecules get within an interval of 
one another which is less—usually much less—than the 
average length of the free paths which the molecules describe 
between the encounters ; which free paths are accordingly 
approximately straight and pursued with unvarying speed 
except so far as they may be slightly influenced by gravity 
or other external cause, or by some excessively minute part 
of the interactions between molecules, if any such survives 
when the interval between molecules gets beyond what we 
may call their encountering distance. 
This ts the Kinetic Theory of Gas as put forward by its 
founder *, and any system of bodies which conforms to this 
definition may be called a Kinetic System. Thus, there are 
in nature as many kinetic systems as there are distinct gases ; 
and moreover, all those models of gas in which the progress 
of events has been studied by mathematicians are each of 
them a kinetic system. So also are the cosmic bodies of 
celestial space, if we eliminate from the definition the 
condition that the masses must be equal ; and, in fact, some 
modification of this clause of the definition is essential even 
as regards gases, inasmuch as in all the gases of nature 
there are found some of the missiles differing in mass from 
others—thus, in diatomic gases ions present themselves with 
masses that seem to be halt the mass of average molecules. 
We may add further details without trespassing beyond 
the domain of theory, i. ¢., while still endeavouring to 
describe events as they occur in nature. Thus we may add 
that elaborate internal events are going on within all these 
missiles, which internal events absorb about one-third of the 
* Clausius’s papers were preceded by a paper by Waterston which 
was presented to the Royal Society in 1845, but which was not then 
published. This paper when it long afterwards came to be printed was 
found to contain a most valuable anticipation of the kinetic theory as 
developed by Clausius. If Waterston’s paper had been printed in due 
course, the Kinetic Theory would probably have been adequately dealt 
with some years sconer, 
