28 Proceedings of Boyal Society of Edinburgh. [sess. 
doctrine of the conservation of energy, in order to afford consistent 
ground for the mechanical interpretation of physical phenomena, 
is that known as the Kinetic theory of gases. 
“In the light of this theory, a gaseous body is a swarm of in- 
numerable solid particles incessantly moving about with different 
velocities in rectilinear paths of all conceivable directions, the 
velocities and directions being changed by mutual encounters at 
intervals, which are short in comparison with ordinary standards 
of duration, but indefinitely long as compared with the duration 
of the encounters. It is readily seen that these motions would 
soon come to an end if the particles were wholly inelastic, or 
imperfectly elastic. For in that case there would be loss of 
motion at every encounter. The assumed perpetuity of the 
motion of the particles therefore leads to the necessity of asserting 
their perfect elasticity. And this necessity results, not merely 
from the peculiar exigencies of the kinetic theory of gases, hut 
also from the principle of the conservation of energy in its general 
application to the ultimate constituents of sensible masses, if these 
constituents are supposed to he in motion. In the case of the 
collision of ordinary inelastic or partially elastic bodies, there is 
a loss of motion which is accounted for by the conversion of the 
motion thus lost into an agitation of the minute parts composing 
the colliding bodies. But in atoms or molecules destitute of parts 
no such conversion is possible, and hence we are constrained to 
assume that the ultimate molecules of a gaseous body are absolutely 
elastic. 
“ The necessity of attributing perfect elasticity to the elementary 
molecules or atoms, in view of the kinetic theory of gases, has been 
expressly recognised by all its founders. £ Gases , 5 says Kroenig, 
‘ consist of atoms which behave like solid, 'perfectly elastic 
spheres, moving with definite velocities in void space . 5 This 
statement is adopted by Clausius, and emphasised by Maxwell, 
the first part of whose essay, ‘ Illustration of the Dynamical 
Theory of Gases , 5 is a treatise on the motions and collisions 
of perfectly elastic spheres. And the highest scientific authorities 
are equally explicit in declaring that the hypothesis of the atomic 
or molecular constitution of matter is in conflict with the doctrine 
of the conservation of energy, unless the atoms or molecules are 
