398 THE QUANTUM THEORYform of the third law was arrived at and accepted only after decades of controversy
and confusion. For the present account, it is important to note the influence of
Einstein's work on this evolution.
On December 23, 1905, Hermann Walther Nernst read a paper at the Goet-
tingen Academy entitled 'On the Computation of Chemical Equilibria from Ther-
mal Measurements.' In this work he proposed a new hypothesis for the thermal
behavior of liquids and solids at absolute zero [N3]. For our purposes, the 1905
hypothesis is of particular interest as it applies to a chemically homogeneous sub-
stance. For this case, the hypothesis states in essence that the entropy difference
between two modifications of such a substance (for example, graphite and dia-
mond in the case of carbon) tends to zero as T — 0. Therefore it does not exclude
a nonzero specific heat at zero temperatures. In fact, in 1906 Nernst assumed that
all specific heats tend to 1.5 cal/deg at T = 0 [N3, N4]. However, he noted that
he had no proof of this statement because of the absence of sufficient low-temper-
ature data. He stressed that it was a 'most urgent task' to acquire these [N3].
Nernst's formidable energies matched his strong determination. He and his col-
laborators embarked on a major program for measuring specific heats at low tem-
peratures. This program covered the same temperature domain already studied
by Dewar, but the precision was much greater and more substances were exam-
ined. One of these was diamond, obviously.
By 1910 Nernst was ready to announce his first results [N5]. From his curves,
'one gains the clear impression that the specific heats become zero or at least take
on very small values at very low temperatures. This is in qualitative agreement
with the theory developed by Herr Einstein. ..'
Thus, the order of events was as follows. Late in 1905 Nernst stated a primitive
version of the third law. In 1906 Einstein gave the first example of a theory that
implies that cv —» 0 as T —> 0 for solids. In 1910 Nernst noted the compatibility
of Einstein's result with 'the heat theorem developed by me.' However, it was
actually Planck who, later in 1910, took a step that 'not only in form but also in
content goes a bit beyond [the formulation given by] Nernst himself.' In Planck's
formulation, the specific heat of solids and liquids does go to zero as T —*• 0 [P2].
It should be stressed that neither Nernst nor Planck gave a proof of the third law.
The status of this law was apparently somewhat confused, as is clear from Ein-
stein's remark in 1914 that 'all attempts to derive Nernst's theorem theoretically
in a thermodynamic way with the help of the experimental fact that the specific
heat vanishes at T = 0 must be considered to have failed.' Einstein went on to
remark—rightly so—that the quantum theory is indispensable for an understand-
ing of this theorem [E8]. In an earlier letter to Ehrenfest, he had been sharply
critical of the speculations by Nernst and Planck [E9].
Nernst's reference to Einstein in his paper of 1910 was the first occasion on
*Simon has given an excellent historical survey of this development [S2j.