which we call Heat. 113 
selves; and secondly, the molecule may vibrate as a whole: 
again, the latter vibrations may consist in oscillations to and fro 
of the centre of gravity, as well as in rotatory oscillations around 
this centre of gravity. In cases where external forces act on the 
body, as in concussions, the molecules may also be permanently 
displaced. 
In the liguid state the molecules have no longer any definite 
position of equilibrium. They can turn completely around their 
centres of gravity; and the latter, too, may be moved completely 
out of its place. The separating action of the motion is not, 
however, sufficiently strong, in comparison to the mutual attrac- 
tion between the molecules, to be able to separate the latter en- 
tirely. Although a molecule no longer adheres to definite neigh- 
bouring molecules, still it does not spontaneously forsake the 
latter, but only under the united actions of forces proceeding 
from other molecules, with respect to which it then occupies the 
same position asit formerly did with respect to its neighbouring 
molecules. In liquids, therefore, an oscillatory, a rotatory, and 
a translatory motion of the molecules takes place, but in such a 
manner that these molecules are not thereby separated from each 
other, but, even in the absence of external forces, remain within 
a certain volume. 
Lastly, in the gaseous state the motion of the molecules entirely 
transports them beyond the spheres of their mutual attraction, 
causing them to recede in right lines according to the ordinary 
laws of motion. If two such molecules come into collision du- 
ring their motion, they will in general fly asunder again with 
the same vehemence with which they moved towards each other ; 
and this will the more readily occur, since a molecule will be 
attracted with much less force by another single molecule than 
by all the molecules which in the liquid or solid state surround it. 
7. The phenomenon of evaporation appearing peculiarly in- 
teresting to me, I have attempted to account for the same in the 
following manner. 
It has been stated above, that in liquids a molecule, during 
its motion, either remains within the sphere of attraction of its 
neighbouring molecules, or only leaves the same in order to take 
up a corresponding position with respect to other neighbouring 
molecules. This applies only to the mean value of the motions, 
however ; and as the latter are quite irregular, we must assume 
- that the velocities of the several molecules deviate within wide 
limits on both sides of this mean value. 
Taking next the surface of a liquid into consideration, I assume 
that, amongst the varied motions to and fro, it happens that 
under the influence of a favourable cooperation of the transla- 
tory, oscillatory, and rotatory motions, a molecule separates itself 
Phil. Mag. 8, 4. Vol. 14, No. 91. Aug. 1857. I 
