THE REALITY OF MOLECULES 95
gave him N = 6.17 X 1023 (!).* The second one makes use of the incompressible
flow of solutions and gave him TV = 2.1 X 1023 , as we saw in the previous section.
The third one, on Brownian motion, gave him a formula but not yet a number.
'May some researcher soon succeed in deciding the question raised here, which is
important for the theory of heat,' he wrote at the end of this paper.** Even though
he did not know the literature, he was right in surmising that the appropriate data
were not yet available. It would soon be otherwise. Incidentally, neither in his
thesis nor in his Brownian motion paper does Einstein mention that in 1905 he
had made not just one but several proposals for determining N. If sparseness of
references to the work of others is typical of his writings, so it is with references
to his own work. He never was a man to waste much time on footnotes.
Einstein was still not done with the invention of new ways for obtaining Avo-
gadro's number. Later in the year, in December, he finished his second paper on
Brownian motion, which contains two further methods for finding N [E8]. In
1907 he noted that measurements of voltage fluctuations give another means for
determining TV [E9]. In 1910 he gave yet another method, critical opalescence
[E10]. He must have realized that the ubiquity of TV would once and for all settle
the problem of molecular reality, as indeed it did.
It was indicated earlier that, as the nineteenth century drew to an end, the
acceptance of the reality of atoms and molecules was widespread, though there
were still some pockets of resistance. Nevertheless, it is correct to say that the
debate on molecular reality came to a close only as a result of developments in the
first decade of the twentieth century. This was not just because of Einstein's first
paper on Brownian motion or of any single good determination of N. Rather, the
issue was settled once and for all because of the extraordinary agreement in the
values of N obtained by many different methods. Matters were clinched not by a
determination but by an overdetermination of TV. From subjects as diverse as
radioactivity, Brownian motion, and the blue in the sky, it was possible to state,
by 1909, that a dozen independent ways of measuring TV yielded results all of
which lay between 6 and 9 X 1023. In concluding his 1909 memoir on the subject,
Perrin [P7, P8] had every reason to state, 'I think it is impossible that a mind free
from all preconception can reflect upon the extreme diversity of the phenomena
which thus converge to the same result without experiencing a strong impression,
and I think that it will henceforth be difficult to defend by rational arguments a
hostile attitude to molecular hypotheses' [P8].f
- Einstein's First Paper on Brownian Motion. Enlarging on an earlier com-
ment, I shall explain next in what sense this first paper on Brownian motion is
- See Section 19b.
**I heed Einstein's remark [E2] that his molecular-kinetic derivation of van 't Hoff's law, also
contained in this article, is not essential to an understanding of the rest of his arguments.
fFor the status of our knowledge about N in 1980, see [D3].