116 Prof. Clausius on the Nature of the Motion 
even the most favourable combination of the several molecular 
motions is not able to overcome this cohesion. 
9. The explanation of the generation and consumption of heat 
accompanying changes of the state of aggregation and of the 
volume of a body, follows immediately, according to the above 
principles, from the consideration of the positive and negative 
work done by the active forces. 
In the first place, let us consider the interior work. 
When the molecules of a body change their relative positions, 
the change may be either in the same direction as that according 
to which the forces resident in the molecules tend to move the 
latter, or in a direction opposite to this. In the first case, a cer- 
tain velocity is imparted by the forces to the molecules during 
their passage from one position to the other, and the vis viva of 
this velocity is immediately transformed into heat ; in the second 
case, and disregarding for the present the action of extraordinary 
foreign forces, it is in virtue of heat that the molecules move in 
directions opposed to the interior forces, and the retardation which 
these molecules thereby suffer through the action of the opposing 
forces appears as a diminution of the motion of heat. 
In the passage from the solid to the liquid state the molecules 
do not, indeed, recede beyond the spheres of their mutual action ; 
but, according to the above hypothesis, they pass from a definite 
and, with respect to the molecular forces, suitable position, to 
other irregular positions, in doing which the forces which tend to 
retain the molecules in the former position have to be overcome. 
In evaporation, the complete separation which takes place 
between the several molecules and the remaining mass evidently 
- again necessitates the overcoming of opposing forces. 
With respect to gaseous bodies, it is evident from what has 
been above remarked concerning the requisites of a perfect 
gaseous state, that as soon as a gas has once attained this state, 
molecular attractions have no longer to be overcome during its 
further expansion, so that during the changes of volume of an 
ideal gas no interior work has to be performed. 
10. Besides the interior, we have in the next place to consider 
the exterior work, and the corresponding change in the quantity 
of heat. 
In the first place, with respect to the changes in the heat of a 
permanent gas subjected to pressure or expanding by overcoming 
a counter-pressure, Kronig has already remarked at the conclu- 
sion of his memoir, that when one of the sides against which the 
molecules of gas strike is itself in motion, these molecules will 
not rebound with velocities equal to those they possessed when 
moving towards the side. If the side approaches the molecules, 
the velocity of the molecules on leaving the same is in general 
