Escape of Gases from Atmospheres. 695 
whole available energy of the gas; and we know that two 
partitions of energy take place—one a partition of energy 
(which probably goes on uninterruptedly) between these 
internal events of the molecules and the events of the ether, 
the other a partition of energy which now and then occurs 
with comparative suddenness between the internal events of 
the molecules and their translational motions. This latter 
transier of energy seems to take place only when two 
molecules are in grip with one another during an encounter, 
and not at every encounter, but only during those which take 
place under certain necessary conditions. If, as seems 
probable, encounters with these special characteristics are 
as rare as those which result in the breaking down of 
molecules into ions, or of those which result in chemical 
reaction in a mixture of equal volumes of chlorine and 
hydrogen, then the infrequency of their recurrence can be 
estimated ; and, in cases in which it has been found possible 
to make the estimate, the infrequency seems to range from 
one out of 109 encounters down to about one in 10”, when 
we pursue the observations so far as they have been recorded. 
It is here that I strongly suspect, though I am not ina 
position to claim that I know, that the mistake has been 
made by Mr. Cook, and by my friend Professor Bryan, who 
both tacitly assume that this partition of energy is a process 
which goes on uninterruptedly, even in the upper parts of the 
atmosphere. Whether the mistake be here or elsewhere 
may as yet be only highly probable; but that a mistake 
exists somewhere in the premisses of the deductive argument 
was placed beyond question by nature when she presented to 
us events that have occurred, or are occurring, which negative 
some of the inferences to which those data lead. We may 
be unable with certitude to put our finger upon the precise 
spot where the mistake came in, but that a mistake has come 
in somewhere can be proved. 
When Maxwell determines his law for the distribution of 
speeds in a kinetic system, he exercises a caution * which has 
not always been observed by his successors, and is careful to 
present the law as the law governing the distribution of 
speeds (not in every, or indeed in : any gas, but) in a kinetic 
system which consists of numberless equal particles, each of 
which is a perfectly rigid and perfectly elastic sphere, after 
an immense number of collisions have taken maki 
assumptions which he afterwards varied in different ways, 
by substituting particles of other forms, or points vepalifene 
* See Maxwell’s ‘Scientific Papers,’ vol. i. p. 380; or Phil. Mag. tor 
January 1860. 
