2 
Transactions of the Royal Microscopical Society. 
velopments into which I shall enter shortly, will show how much 
we ought to ascribe to this really ingenious, hut much too in- 
complete idea. 
The Brownian motion of minute particles in suspension in 
liquids, is a movement of oscillation and of vibration, in situ, 
that is quick, irregular, and continuous. We do not remark there 
either translation or locomotion properly so called. The orientation 
of the oscillatory motion passes briskly, and without following any 
law, from one direction to another. In the cellules of the epi- 
dermis, the pigmentary granulations having less than five or six 
thousandths of a millimeter in diameter, are animated with this 
motion of vibration ; the grains of chlorophyll in the green cellules, 
and probably all the cellular granulations whose surrounding liquid 
is not solidified, likewise manifest this movement. It is observed 
also in gold particles, in little grains of iridium, platinum, coal, 
lime, &c., &c., in milk globules, and more generally in all viscous 
globules immersed in water or in liquids that have little resistance. 
The phenomenon occurs also in the little gas-bubbles imprisoned 
in a liquid ; for example, in the air-bubbles which are so easily 
formed by agitating soap and water. The Brownian motion is 
more active in heated liquids than in those of a low temperature. 
Supposing equal diameters, the oscillatory displacement is more 
rapid and more extended in fatty granulations than in metallic 
granulations whose density is very great. The duration of the 
phenomenon may be said to be without limit : M. Bobin possesses 
aqueous preparations of charcoal dust, made more than twenty 
years ago, in which the Brownian motion still continues to manifest 
itself. 
In this respect, quartz rocks are yet more remarkable : the 
Brownian motion has been going on in them for millions of years. 
In fact, it is not a rare thing to find, in the quartz of geological 
strata, liquid cavities containing a gas-bubble in a state of perpetual 
agitation. It is a little bubble of vapour produced by the with- 
drawal of the liquid, and which the Brownian motion carries 
hither and thither into all the recesses of its transparent prison. 
Of all the physical phenomena that the microscopic study of rocks, 
so fruitful of surprising results, has revealed to us, this fact, first 
observed by Mr. Sorby, is certainly one of the most beautiful. 
My intention in this note is to show, that all the Brownian 
motions of small masses of gas and of vapour in suspension in 
liquids, as well as the motions with which viscous granulations 
and solid particles are animated in the same circumstances, proceed 
necessarily from the molecular heat motions, universally admitted, 
in gases and liquids, by the best authorized promoters of the 
mechanical theory of heat. I have been led to consider the phe- 
nomenon from this point of view, from the study I have made of 
