NATURE 
263 
THURSDAY ;* MAY “1a; 1974. 
RECENT EXTENSIONS OF THE QUANTUM 
HYPOTHESIS. 
Die Theorie der Strahlung und der Quanten. Mit 
einem Anhange tber die Entwicklung der 
OQuantentheoric vom Herbst 1911 bis sum Som- 
mer 1913.’.° Edited by A. Eucken. Pp. xii+ 
405. (Halle) a. S.:~ Wilhelm Knapp,” 1914.) 
Price 18.60 marks. 
A. SHORT account was given in NaTuRE in 
4 November, rgt1, of a meeting of the prin- 
cipal authorities on radiation questions held in 
the autumn of that year in Brussels, under the 
auspices of M. Ernest Solvay. The present work 
is the German edition of the papers read at that 
congress, together with the discussions which took 
place on them. In the course of the last two 
years the subject has developed very considerably, 
and the opinions expressed at Brussels would only 
give a very incomplete view of the present state 
of the theory, but this defect has been corrected 
by the editor, Prof. Eucken, who provides at the 
end of the book a fairly detailed sketch of the 
chief advances up to the summer of 1913. 
This section provides extraordinarily interesting 
reading. Though no solution has yet been found 
of the central problem, several new phenomena 
have been brought under the quantum régime, and 
the position of the theory of specific heats under 
that régime has been completely altered. The 
older theory of Einstein and Nernst (an account 
of which is given in the earlier part of the book), 
supposed that every atom ina solid vibrates with 
a certain definite frequency, and so by the quan- 
tum principle can only take up energy in certain 
definite amounts. The resulting value for the 
specific heat agrees only very roughly with ex- 
periment at low temperatures. The new theory, 
developed independently by Debye and by Born 
and Karman, applies the quantum principle not 
to the separate atoms, but to the elastic waves 
which can be propagated through the body. The 
agreement with experiment is very greatly im- 
proved, and there can be little doubt that the work 
provides the right basis for a theory, though the 
mathematical difficulties have so far prevented its 
being worked out completely. Debye’s applica- 
tion of the quantum principle directly to waves 
instead of merely to vibrating electrons is one of 
the most important changes of aspect which have 
come over the subject. Since energy in a wave 
is not localised in one spot, the new aspect makes 
the physical comprehension of the quantum even 
harder than it was before, but in spite of this — 
there is a gain in generality, and it should prob- 
NON23245 VOL; .93) 
| 
ably be counted as a distinct advance towards the 
final elucidation of the problem. 
Another question which has become very pro- 
minent is ‘‘Nullpunktsenergie ’—residual energy 
at the absolute zero of temperature. This first 
arose in connection with Planck’s second radiation 
hypothesis, according to which a vibrating elec- 
tron absorbs energy following the ordinary laws, 
| but can only emit it when its total energy has 
reached one of a definite series of values. Thus 
near the absolute zero a vibrator may have quite 
a finite amount of energy, since it cannot emit at 
all the energy which it is slowly absorbing. Ac- 
cording to a very important paper by Poincaré— 
almost his last published work—there is grave 
difficulty in accounting for the observed radiation 
formula in this way; but in spite of this the ques- 
tion of residual energy has been the subject of a 
good deal of discussion, ,and it has been invoked 
with some success though in a very speculative 
way, to account for several phenomena. In con- 
sidering the evidence on these points Prof. Eucken 
concludes that each separate one might be ex- 
plained in another way, but that the sum of 
all gives some probability in favour of the ex- 
istence of residuai energy at the absolute zero. 
According to a suggestion of Debye the reflection 
of X-rays may be made to throw light on this, 
since it may be possible to discover in what way 
the atoms of the reflecting crystal vibrate. So 
_ perhaps this important question may be decided 
soon. 
The most striking development, to which Prof. 
Eucken refers, is the application of the quantum 
to the rotation of gas-molecules. Hitherto it had 
only met with success when applied to vibrations. 
In a rotating gas-molecule we have a periodic, but 
not a vibrational motion, and it must be of funda- 
mental importance that the quantum applies to 
this. The most remarkable result of all is the 
work of Eva von Bahr, who finds that the absorp- 
tion of infra-red light by water-vapour may be 
taken to indicate that the molecules are rotating 
only with multiples of two definite angular veloci- 
ties. By the use of the quantum it is possible to 
calculate two moments of inertia for the molecule, 
| and the values deduced are of the size which would 
be expected from its known dimensions. When 
| further developed this work may be expected to 
throw light not only on the meaning of the quan- 
tum, but also on the structure of the molecule. 
It is unfortunate that the book came out Just 
; too early to include a mention of Bohr’s theory of 
spectra. This theory is very speculative, but un- 
like any of the previous theories it does give a 
simple reason for the observed_series in spectra. 
Regd nlial these 
Perhaps his most striking 
oe 
MAY 23 10 
