6 Transactions of the Royal Microscopical Society. 
ning of this article, has only given an incomplete and wholly in- 
sufficient explanation of the phenomenon. 
If the molecules in the mass of liquids are only subject to single 
vibratory motions, the displacing of the centre of gravity of the 
Brownian particles, consisting of thousands of molecules, would 
always remain imperceptible. The influence of the motion of trans- 
lation of the liquid molecules acting on the surface of the particles, 
permits us to explain easily all displacements hitherto observed. 
In fact, just as on the surface of a liquid * “ it is possible that a 
molecule, by a favourable concourse of motions of translation, of 
oscillation, and of rotation, be separated with such violence from 
the rest of the neighbouring molecules, that before all its velocity 
is destroyed by the attractive force of the last-mentioned molecules, 
it is beyond their active sphere, and continues to move in the sur- 
rounding space lying beyond the reach of the liquid,” thus pro- 
ducing the phenomenon of vaporation ; so also in the mass of the 
liquid, it is not only possible, but even necessary, that a favourable 
concourse of the motion of translation, of oscillation, and of rotation 
produce on the different parts of the surface of the Brownian particles 
a pressure endowed with an exceptional intensity. 
The maxima of pressure, sufficiently great on the particles of 
large dimensions to produce equilibrium, are not so on the Brownian 
particles, at least, I may be allowed to suppose it. This supposition 
is quite legitimate, since we only treat of isolated points belonging 
to the surface of the particle, and not to the entire volume or to the 
exterior surface. 
The particles are then drawn with all the energy and irre- 
gularity of movement which the resultant of forces, remarkably 
variable both in intensity and direction, inevitably produces. As 
to the pressure which is not the result of the favourable reunion of 
circumstances of which there is question here, they possess the same 
equilibrium on the Brownian particles as on globules and granula- 
tions of much larger dimensions. 
Mr. Sorby’s Observations on M. Delsaulx' Paper. 
I have read the paper on moving particles with very great 
interest, since it appears to me that the author has suggested the 
best explanation yet propounded. When my attention was first 
directed to the movement of the bubbles in fluid cavities, I could 
not help thinking that it was in some way or other connected 
with those movements which are supposed to he constant in the 
ultimate particles of matter. The chief difficulty was to imagine 
how they could in any case be slow enough and large enough to be 
* Clausius, p. 194. 
