THE REALITY OF MOLECULES 935d. Eleven Days Later: Brownian Motion*- Another Bit of Nineteenth Century History. During the nineteenth century,
it had become clear from experiments performed in various laboratories that
Brownian motions increase with decreasing size and density of the suspended par-
ticles (10~^3 mm is a typical particle radius above which these motions are hardly
observable) and with decreasing viscosity and increasing temperature of the host
liquid. Another important outcome of this early research was that it narrowed
down the number of possible explanations of this phenomenon, beginning with
Brown's own conclusion that it had nothing to do with small things that are alive.
Further investigations eliminated such causes as temperature gradients, mechan-
ical disturbances, capillary actions, irradiation of the liquid (as long as the result-
ing temperature increase can be neglected), and the presence of convection currents
within the liquid. As can be expected, not all of these conclusions were at once
generally accepted without controversy.
In the 1860s, the view emerged that the cause of the phenomenon was to be
found in the internal motions of the fluid. From then on, it did not take long before
the more specific suggestion was made that the zigzag motions of the suspended
particles were due to collisions with the molecules of the fluid. At least three phys-
icists proposed this independently: Giovanni Cantoni from Pavia and the two Bel-
gian Jesuits Joseph Delsaulx and Ignace Carbonelle. Of course, this was a matter
of speculation rather than proof. 'Io penso che il moto di danza delle particelle
solide ... possa attribuirsi alle different! velocita che esser devono ... sia in coteste
particelli solide, sia nelle molecole del liquido che le urtano da ogni banda,' wrote
Cantoni [C6].** '[Les] mouvements browniens. .. seraient, dans ma maniere de
considerer le phenomene, le resultat des mouvements moleculaires calorifiques du
liquide ambiant,' wrote Delsaulx [D5].f
However, these proposals soon met with strong opposition, led by the Swiss
botanist Carl von Naegeli and by William Ramsey. Their counterargument was
based on the incorrect assumption that every single zig or zag in the path of a
suspended particle should be due to a single collision with an individual molecule.
Even though experiments were not very quantitative at that time, it was not dif-
ficult to realize that this assumption led to absurdities. Nevertheless, the expla-
- Einstein's papers on Brownian motion as well as the 1906 paper have been collected in a handy
little book by Fiirth [Fl, F2]. A useful though not complete set of references to nineteenth century
experimental work and theoretical speculation can be found in a paper by Smoluchowski [S3]; see
also [B8] and [N4].
**I believe that the dancing motion of the solid particles ... can be attributed to the different veloc-
ities which ought to be ascribed. .. either to the said solid particles, or to the molecules of the liquid
which hit [these solid particles] from all directions.
fin my way of considering the phenomenon, the Brownian motions should be the consequence of
the molecular heat motions of the ambient liquid.