110 Prof. Clausius on the Nature of the Motion 
3. In one and the same gas the translatory motion of the 
whole molecules will always have a constant relation to the 
several motions which, in addition to the above, the constituents 
of the molecules likewise possess. For brevity I will call the 
latter the motions of the constituents. 
Conceive a number of molecules whose constituents are in 
active motion, but which have no translatory motion. It is evi- 
dent the latter will commence as soon as two molecules in con- - 
tact strike against each other in consequence of the motion of 
their constituents. The translatory motion thus originated will 
of course occasion a corresponding loss of vis viva in the motion 
of the constituents. On the other hand, if the constituents of a 
number of molecules in a state of translatory motion were mo- 
tionless, they could not long remain so, in consequence of the 
collisions between the molecules themselves, and between them 
and fixed sides or walls. It is only when all possible motions 
have reached a certain relation towards one another, which rela- 
tion will depend upon the constitution of the molecules, that 
they will cease mutually to increase or diminish each other. 
When two molecules whose constituents are in motion come 
into collision they will not rebound, like two elastic balls, accord- 
ing to the ordinary laws of elasticity ; for their velocities and diree- 
tions after collision will depend, not only upon the motion which 
the whole molecules had before impact, but also upon the motion 
of those constituents which are nearest each other at the moment 
of collision. After the equalization of the several motions, how- 
ever, when the translatory motion is, on the whole, neither m- 
creased nor diminished by the motions of the constituents, we 
may, in our investigation of the total action of a great number 
of molecules, neglect the irregularities occurring at the several 
collisions, and assume that, in reference to the translatory mo- 
tion, the molecules follow the common laws of elasticity. 
4, The explanation of the expansive force of gases and its 
dependence upon volume and temperature, as given by Krénig, 
suffers no essential modification through the imtroduction of 
other motions. The pressure of the gas against a fixed surface 
is caused by the molecules in great number continually striking 
against and rebounding from the same. The force which must 
thence arise is, in the first place, by equal velocity of motion in- 
versely proportional to the volume of the given quantity of gas ; 
and secondly, by equal volume proportional to the vis viva of the 
translatory motion: the other motions do not here immediately 
come into consideration. 
On the other hand, from Gay-Lussac’s law we know that, 
under constant volume, the pressure of a perfect gas increases 
in the same ratio as the temperature calculated from —273° C., 
