494 
evidence poinung toward a general flotation of the 
crust near to equilibrium be reconciled with this other 
evidence of great rigidity and strength? It has been 
supposed that a mobile zone may explain the apparent 
contradiction, but the necessity of postulating such a 
zone becomes greater as the accumulated evidence of 
weakness on one hand, of strength on the other, 
diverges more and more. By means of a study of the 
areas of the surface loads and their degree of depar- 
ture from isostatic equilibrium this zone is located far 
deeper than other estimates have placed it, the level 
of minimum strength being thought to lie as much 
as 150 to 200 miles deep. The maximum strength is 
probably at a depth of ten to twenty miles, and falls 
off rapidly below. 
“The Geologic and Biologic Results of a Study 
of the Tertiary Floras of South-eastern North 
America’’ were presented by Prof. E. W. Berry, of 
Johns Hopkins University. The results of many years of 
study of the rich Tertiary floras of south-eastern North 
America were announced for the first time. Their 
botanical relationships and their bearing on the evolu- 
tion of types and upon geographical distribution were 
summarised. The studies have afforded for the first 
time fossil floras of fixed stratigraphic position for 
comparison with the floras of the Rocky Mountain 
province on the border between the Cretaceous and 
Tertiary, the age of which has caused much con- 
troversy. They also afford means for correlation with 
the type of section of the Paris basin. 
During the afternoon of April 24 a portrait of the 
late Dr. S. P. Langley, who was a former vice- 
president, was presented to the society by Dr. C. Adler 
on behalf of a number of members. On April 25 the 
following new members were elected as the result of 
balloting,—Mr. C. G. Abbot, Washington; Dr. J. W. 
Bright, Baltimore; Dr. B. M. Davis, Philadelphia ; 
Dr.,T. McCrae, Philadelphia; Dr. W. D. Matthew, 
New York; Dr. A. G. Mayer, Washington; Dr. 
5:2 J-, Meltzer, (News: York« Dr, J..iC) Merceaan 
Berkeley, Cal.; Prof. R. A. Millikan, Chicago; Prof. 
W. A. Noyes, Urbana, Ill.; Dr. Stewart Paton, 
Princeton; Dr. R. M. Pearce, jun., Philadelphia; Dr. 
P. C. Ricketts, Troy; Dr. Harold A. Wilson, F.R.S., 
Houston; Dr. F. E.° Wright, Washington; Dr. 
Shibasaburo Kitasato, Tokyo; Prof. H. Kamerlingh 
Onnes, Leyden; and Dr. Vito Volterra, Rome. 
At the concluding session of the meeting a medallion 
portrait of the late Sir Joseph D. Hooker was un- 
veiled by Prof. W. G. Farlow, of Harvard University. 
A-RAYS AND CRYSTALLINE 
STROCTURE.» 
qawo years have gone by since Dr. Laue made his 
surprising discovery of the interference effects 
accompanying the passage of X-rays through crystals. 
The pioneer experiment has opened the way for many 
others, and a very large amount of work, theoretical 
and practical, has now been done. As the preliminary 
exploration of the new country has proceeded our 
first estimate of its resources has grown continuously ; 
we have learnt many things which help us to a better 
understanding of phenomena already familiar, and 
we have seen avenues of inquiry open out before us 
which as yet there has been little time to follow. 
The work is full of opportunities for exact quantita- 
tive measurement, where precision is sure to bring its 
due reward. There is enough work in sight to absorb 
the energies of many experimenters, and there is sure 
to be far more than we can see. When we consider 
the wideness of the new field, the quality and quan- 
tity of the work to be done in it, and the importance 
1 Discourse delivered at the Royal Institution on June 5 by Pro® W. H. 
Bragg, F R.S. 
NOW :23 32, VOL. ©3)| 
NATURE 
| irreconcilable 
[JuLy 9, 19144) 
of the issues, we are scarcely guilty of over-statement 
| if we say that Laue’s experiment has led to the 
development of a new science. 
The experiment itself—to put it very briefly—con-— 
stitutes a proof that X-rays consist of extremely short 
zther waves. In order to appreciate the value of this 
demonstration, we must bear in mind the present 
conditions of our knowledge of the laws of radiation 
in general. 
whole matter stood when the new work was begun. 
When X-rays were first discovered eighteen years 
ago it was soon pointed out that they might consist 
of electromagnetic disturbance of the zther analogous 
to those supposed to constitute light. It was true 
that the new rays seemed to be incapable of reflec- 
tion, refraction, diffraction and interference which 
were familiar optical phenomena. But it was pointed 
out by Schuster. (Nature, January 23,. 1896) 
that these defects could be explained as natural 
consequences of an extremely small wave-length. 
The positive evidence consisted mainly in the 
knowledge that the impact of the electrons on the 
antikathode of the X-ray bulb ought to be the occasion 
of electromagnetic waves of some sort, and in the 
discovery by Barkla that the X-rays could be polarised, 
which last is a property also of light. 
As experimental evidence accumulated, a number of 
results were found which the electromagnetic theory 
was unable to explain, at least in a direct and simple 
manner. They were mainly concerned with the trans- 
ference of energy from place to place. In some way 
or other the swiftly moving electron of the X-ray 
bulb transfers its energy to the X-ray, and the X-ray 
in its turn communicates approximately the same 
quantity of energy to the electron which originates 
from matter lying in the track of the X-ray, and 
which is apparently the direct cause of all X-ray 
effects. Experiment seemed to indicate that X-ray 
energy travelled as a stream of separate entities or 
quanta, the energy of the quantum differing accord- 
ing to the quality of the X-ray. It looked at one 
time as if it might be the simplest plan to deny the 
identity in nature of X-rays and light, to describe the 
former as a corpuscular radiation, and the latter as 
a wave motion. Otherwise it seemed that the electro- 
magnetic hypothesis would be torn to pieces in the 
effort to hold all the facts together. 
But it appeared on a close examination of light 
phenomena also, though in much less obvious fashion, 
that the very same effects occurred which in X-rays 
were so difficult to explain from an orthodox point of 
view. In the end it became less difficult to deny the 
completeness of the orthodox theory, than the identity 
in nature of light and X-rays. Modern work on the 
distribution of energy in the spectrum, and the de- 
pendence of specific heat upon temperature have also led 
independently to the same point of view. It has been 
urged with great force by Planck, Einstein, and others 
that radiated energy is actually transferred in definite 
units or quanta, and not continuously: as if we had 
| to conceive of atoms of energy as well as of atoms of 
matter. Let it be admitted at once that the quantum 
theory and the orthodox theory appear to stand in 
opposition. Each by itself correlates 
great series of facts; but they do not correlate the 
same series. In some way or other the greater theory 
must be found, of which each is a partial expression. 
The new discovery does not solve our difficulty at 
once, but it does two very important things. In the 
first place, it shows that the X-rays and light are 
identical in nature; in fact, it removes every differ- 
ence except in respect to wave-length. The question 
as to the exact place where the difficulty lies is 
decided for us; we are set the task of discovering 
how a continuous wave motion, in a continuous 
Let us consider very shortly how the ¥ 
