DEC 1 1 1901 
THE 
I. — Thermo-dynamic Origin of the Brownian Motions. 
By the Rev. Joseph Delsaulx, S.J. 
{Read before the Royal Microscopical Society, June 6, 1877 ). 
The Brownian, or molecular motions, have been hardly known 
more than fifty years. Robert Brown announced, in 1829, that 
when extremely minute solid particles, either organic or inorganic, 
are found suspended in pure water, or in an aqueous fluid, they 
display certain motions whose cause he was unable to discover, and 
which, by their irregularity and apparent independence, resembled 
to a remarkable degree the less rapid motions of the simplest in- 
fusoria. The smallest of these particles he called active molecules.* 
The motions discovered by Robert Brown in minute particles, and 
for that reason called Brownian motions, have since been observed 
by all naturalists. In fact, there is not one amongst them but 
must have been struck by the strangeness, the persistence, and the 
frequent apparition of these molecular motions in the field of the 
microscope ; not one, I fancy, who has not tried to raise up, were 
it only by a corner, the veil which nature has cast upon the secret 
of their origin. Hitherto, it must be confessed, all their efforts 
have been fruitless : the sphinx has kept his enigma. 
A friend of mine has, I think, approached the nearest to the 
truth in investigating this matter. His opinion, the fundamental 
idea of which has been put in print, t may be expressed in these 
terms : “ Every free particle, the molecules of which remain as- 
sociated by their mutual actions as in the liquids and the solids, or 
by an external pressure as in the gas-bubbles in the mass of a liquid, 
must oscillate incessantly, if it is sufficiently small. These oscilla- 
tions are a necessary result of the molecular vibrations which con- 
stitute heat ; because each molecule, in vibrating, tends to displace 
the centre of gravity of the body to which it belongs. If this dis- 
placement is not commonly produced in the bodies we observe, it is 
because the effect of one, owing to the immense number of molecules, 
is always neutralized by that of another.” } The theoretical de- 
* Ch. Robin, ‘ Traite <lu Microscope,’ p. 526. 
t ‘ Bulletin dc l’Acadcmie Royal de Belgique,’ t. xli. p. 410. 
t Ibid. t. xli. p. 410. 
VOL. XVIII. 
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