88 
NATURE 
[Marcu 25, 1915 

discussion of recent work upon their reflection 
from the interior of crystals. 
Although we do not agree with the footnote to 
p- 165, the application of X-rays to medicine is 
mentioned with a moderation which will be under- 
stood and appreciated by those who are expert in 
that branch of the subject. We have no hesita- 
tion in recommending that a copy of the book 
should be in the library of every medical practi- 
tioner who desires to master the fundamental 
ideas underlying the properties and actions of the 
radiation which he is putting to good use in the 
alleviation or cure of disease. 
(2) The study of the clinical application of 
X-rays is more often than not approached from a 
purely medical point of view, and some there are 
who think that a wider scientific knowledge, 
especially of physics, should play a large part in 
the early training of the student of radiology. It 
is probable that with them at least this book will 
find no favour; indeed, were it not for the fact 
that the author states specifically in his preface, 
for whom this book is intended some doubt upon 
the point might reasonably have arisen. 
It must be borne in mind, however, that we 
have to deal here with a manual intended to meet 
the “needs of the United States army,” and, 
judging by the somewhat limited knowledge of the 
subject considered sufficient in our own service, 
a student who assimilates the contents of this 
work might be counted as one possessing excep- 
tional qualifications. 
If “brevity is the soul of wit,” it may also be 
the source of much error, and the inadequate treat- 
ment of the elements of electricity and magnetism 
condensed into the first nine pages contains some 
misleading statements. From chapter iii. on- 
wards, however, there will be found many useful 
practical hints as to radiographic technique, and 
the tersely written survey (which certainly covers 
a wide field) affords a suitable framework upon 
which to build by experience if supplemented by 
the study of more complete works dealing with 
the subject. 
(3) It is noteworthy that in this small handy 
volume Mr. Crowther has succeeded in giving a 
remarkably complete survey of work dealing with 
the physics of the electron and atomic structure. 
The treatment of the subject is naturally in- 
fluenced very largely by the trend of researches 
carried out during the last few years at the Caven- 
dish Laboratory, and the work therefore stands 
as an authoritative and connected account of the 
most important recent results which have emanated 
‘from Cambridge under the stimulating leadership 
of Prof. Sir J. J. Thomson. The new method of 


simultaneous deviation of positive particles by an 
electrostatic and magnetic field, is dealt with in 
considerable detail and the apparatus illustrated 
by photograph and diagrams. There are also 
chapters devoted to the nature and size of elec- 
trons, their group stability, cohesion, and adhe- 
sion, as well as the bearing of modern physics 
upon valency and other problems once thought to 
be the exclusive preserves of the chemist. In the 
midst of great detail the author has nevertheless 
marked out clearly the general lines upon which 
| this important subject has rapidly developed in 
recent years, and has brought together into a con- 
nected whole the results of work from many 
sources. It is difficult to select any particular 
section for special mention where all are so well 
done, but the chapter dealing with the atom in 
vibration is certainly of particular interest. 
The author then shows the far-reaching signi- 
ficance of these new experimental results by 
directing attention to their bearing upon the com- 
plex “molecular theory of matter”’ as applied to 
solids, liquids, and gases. In the later pages he 
refers to the kinetic theory of heat, the size of 
atoms, and many other questions of wide interest 
and importance at the present day. The book, so 
well conceived, is a veritable mine of information. 
It will be read by physicists with only one regret 
—the author has not included even the briefest 
index. CHESS ae 

OUR BOOKSHELF. 
Economic Cycles: Their Law and Cause. By 
Prof. H. L. Moore. Pp. vilit149. (New 
York: The Macmillan Co.; London: Mac- 
millan and Co., Ltd., 1914.) Price 8s. 6d. net. 
In this volume Prof. Moore has undertaken to 
test statistically the theory that the fundamental 
cause of that curious swing in trade, which 
results in alternate periods of depression and ex- 
pansion instead of steady growth or decline, is 
a corresponding cycle in the weather, operating 
through its effect on the crops. Rainfall data 
for the Ohio valley are subjected to harmonic 
analysis, and the periodogram shows principal 
periods of thirty-three and eight years. These 
periods and their semiharmonics applied to the 
statistics of yield and production for some of the 
principal crops suggest that they will also account 
for the principal fluctuations in quantity, as might 
be expected from the high correlation between 
the yield and the rainfall of the months that are 
critical for each crop. Changes in yield are also 
highly correlated with changes in price for the 
several crops, and hence the general relation of 
the crop-cycle to the cycle of prices and trade in 
general—though matters are not so simple as 
might be thought at first sight, for in some manu- 
facturing industries, e.g., the production of pig- 
analysis, for instance, which depends upon the | iron, price and production rise and fall together. 
NO. 2369, VOL. 95] 
