Supplement to ‘ Nature,” 
June 9, 1923 ill 

New Methods of Crystal Analysis and their. Bearing on Pure and Applied Science. 
By Sir Wi11am Bra@c, K.B.E., FRS, 
T is one of the most fascinating of all studies to 
trace back the properties of the substances that 
we see round about us to the manner and the details 
of their underlying structure. . There are in the world, 
or indeed the universe, a certain number of different 
kinds of the atoms of which all things are made. We 
know of rather more than ninety in all. The science 
of radioactivity has brought to our notice atoms dis- 
tinguished by special powers of emitting radiations, 
but the list is not really increased thereby. Every- 
thing we see round about us, or are aware of when 
perhaps we cannot see, is built up by joining together 
these atoms in various ways: and all the properties of 
substances, their infinite complications, powers, and 
beauties, are associated with the properties of the 
atoms even before construction is begun. It is surely 
no wonder that we try to find out how this is done. 
Chemistry itself has its origin in this quest. One 
of its early successes was the explanation—incomplete, 
no doubt—of the part played by oxygen in the act of 
burning or rusting. As chemistry has grown to its 
present magnitude all its findings have related to the 
part played by this or that atom or combination of 
atoms in determining the properties of various sub- 
stances. The methods of chemistry are founded on 
the study of the behaviour of crowds. The smallest 
portion of any substance handled in the laboratory 
contains billions of atoms; and the properties of the 
individual are inferred from the treatment of gross 
aggregates. The chemist mixes together two liquids 
in certain proportions, observes, tests, and weighs the 
results; and he infers that atoms already grouped 
in certain combinations are ready to change to fresh 
groupings. From his weighings he finds the proportions 
in which the atoms break with one another and re- 
combine. He observes and measures their readiness 
to change partners. Sometimes the exchange is so 
rapid that energy is liberated with explosive violence. 
Sometimes it is so slow that it must be hurried, either 
by the application of warmth or by other means, the 
quaintest of which is the action of a catalyst, a third 
body which promotes a new grouping without being 
finally concerned in it; as the chaperone of bygone 
days effected the introduction between two people 
anxious to meet each other and then effaced herself. 
The science of radioactivity takes up the study of 
the atom in a totally different way. It finds that 
sometimes atoms are endowed with movement so 
rapid that the individual has enough energy to make 
its own mark. In the spinthariscope of twenty-five 
1 Sixth Trueman Wood Lecture, delivered at the Royal Society of Arts on 
January 24. 
atom 
‘years ago Sir William Crookes showed the separate and 
Visible flashes which were made when a succession of 
helium atoms, shot out from radium, struck a phos- 
phorescent screen. Each impact made its little flash 
of light just as when a pebble is dropped at night into 
@ phosphorescent sea. This is a typical experiment 
belonging to the science of radioactivity, typical in 
that it deals with the individual and not with the 
crowd. In this science there is very little concern 
with the combinations of atoms. It leads more to a 
Study of the nature of the internal structure of the 
: that is why if we wish to understand the atom’s 
inner mechanism we turn to the work which J. J. 
Thomson, Rutherford, Aston, and others are doing. 
The new methods with which I deal here attack the 
- question from yet another aspect, based on the recog- 
nition of the properties of crystals on one hand and 
of X-rays on the other. 
A crystal that has grown without disturbance pre- 
sents surfaces of brilliant polish which make with each 
other angles of characteristic and invariable magnitude. 
Sometimes one face grows abnormally as compared 
to others, on account, it may be, of some disposing 
cause in the circumstances in which the crystal was 
formed, but in crystals of the same substance the 
angles between corresponding faces are always exactly 
the same. There are not, usually, many different 
kinds of faces on a crystal. Often on careful examina- 
tion it is found that there are not more than three or 
four. If we examine specimens of the same crystal 
which seem at first sight to differ in form, we find that 
the difference is nothing more than an unequal develop- 
ment of the various types of face. An outward pre- 
sentment so simple as this must imply a like simplicity 
in internal design. There must be a unit of pattern 
which contains but few atoms and, repeated again and 
again through space, makes up the whole crystal. The . 
idea has ‘long been familiar to the crystallographer, 
but he could not push the corresponding interpreta- 
tion to its limits: he had no clue to guide him, no 
methods of examining the actual details of the design. 
The reason of the failure is not difficult to understand : 
the details were too fine to be distinguished under 
the most powerful microscope. Nor is this a mere 
question of a lack of technical skill which might be 
removed at some future time. It will never be possible 
to see the arrangements of atoms in a crystal. 
When we say that we see any particular thing, 
what we really do is to observe some change which 
the thing has made in the light waves which reach our 
eyes after they have been reflected or scattered or in 
