490 
NATURE 
[JuLy 9, 1914 
telescope by an ionisation chamber and an electroscope. L ewe or three or any number of atoms of each 
In use a fine pencil of X-rays is directed upon the 
crystal which is steadily turned until a reflection leaps 
out; and the angle of reflection is then measured. 
If we use different crystals or different faces of the 
same crystal, but keep the rays the same we can 
compare the geometrical spacings of the various sets 
of planes. If we use the same crystal always, but 
vary the source of X-rays we can analyse the latter, 
measuring the relative wave-lengths of the various 
constituents of the radiation. 
We have thus acquired a double power :— 
(1) We can compare the intervals of spacing of 
the atoms of a crystal or of different crystals, along 
various directions within the crystal; in this way 
we can arrive at the structure of the crystal. 
(2) We can analyse the radiation of an X-ray bulb; 
in fact, we are in the same position as we should 
have been in respect to light if our only means of 
analysing light had been by the use of coloured 
glasses, and we had then been presented with a 
spectrometer, or some other means of measuring wave- 
length exactly. 
We now come to a critical point. If we knew the 
exact spacings of the planes of some one crystal we 
could now by comparison find the spacings of all other 
crystals and measure the wave-lengths of all X-radia- 
tions. Or if we knew the exact value of some one 
wave-length we could find by comparison the values 
of all other wave-lengths, and determine the spacings 
of all crystals. But as yet we have no absolute value 
either of wave-length or of spacings. 
The difficulty appears to have been overcome by 
W. L. Bragg’s comparison of the reflection effects 
in the case of rock-salt or sodium chloride and sylvine 
or potassium chloride. These two crystals are known 
to be ‘isomorphous’; they must possess similar 
arrangements of atoms. Yet they display a striking 
difference both in the Laue photograph and on the 
spectrometer. The reflections from the various series 
of planes of the latter crystal show spacings con- 
sonant with an arrangement in the simplest cubical 
array. The smallest element of pattern is a cube at 
each corner of which is placed the same group, a 
single atom or molecule or group of atoms or mole- 
cules. In the case of rock salt, the indications are 
that the crystal possesses a structure intermediate 
between the very simple arrangement just described 
and one in which the smallest element is a cube 
having a similar group of atoms or molecules at every 
corner and at the middle point of each face. The 
arrangement is called by crystallographers the face- 
centred cube. The substitution of the sodium for the 
potassium atom must transform one arrangement into 
the other. 
This can be done in the following way, if 
we accept various indications that atoms of equal 
weight are to be treated as equivalent. Imagine an 
elementary cube of the crystal pattern to have an atom 
of chlorine at every corner and in the middle of each 
face, and an atom of sodium or potassium as the case 
may be, at the middle point of each edge and at the 
centre of the cube. We have now an arrangement 
which fits the facts exactly. The weights of the 
potassium and chlorine atoms are so nearly the same 
as to be practically equivalent, and when they are 
considered to be so, the arrangement becomes the 
simple cube of sylvine. But when the lighter sodium re- 
places the potassium, as in rock-salt, the arrangement 
is on its way to be that of the face-centred cube, and 
would actually become so were the weight of the 
sodium atoms negligible in comparison with those of 
chlorine. 
Of course, 
the same result would follow 
NOW2 332) VOL oa 
were 
sort to take the place of the single atom, provided 
the same increase was made in the number of the 
atoms of both sorts. We might even imagine two 
sorts of groups of chlorine and metal atoms, one 
containing a preponderance of the former, the other 
of the latter, but so that two groups one of each kind 
contained between them the same proportion of chlorine 
and metal as the crystal does. We must merely have 
two groups which differ in weight in the case of rock- 
salt and are approximately equal in weight in the case 
of sylvine. But it was best to take the simplest sup- 
position at the outset; and now the evidence that the 
right arrangement has been chosen is growing as 
fresh crystals are measured. For it turns out that in 
all crystals so far investigated, the number of atoms 
at each point must always be the same. Why, then, 
should it be more than one? Or, in other words, 
if atoms are always found in groups of a certain 
number, ought not that group to be called the atom? 
So soon as the structure of a crystal has been found 
we can at once find by simple arithmetic the scale on 
which it is built. For we know from other sources © 
the weight of individual atoms, and we know the 
total weight of the atoms in a cubic centimetre of the 
crystal. In this way we find that the nearest distance 
between two atoms in rock-salt is 2-81x10—-§ cm., 
which distance is also the spacing of the planes 
parallel to a cube face. 
From a knowledge of this quantity the length of 
any X-ray wave can be calculated at once so soon as 
the angle of its reflection by the cube face has been 
measured. In other words, the spectrometer has now 
become a means of measuring the length of waves of 
any X-radiation and the actual spacings of the atoms 
of any crystal. 
From this point the work branches out in several 
directions. It will not be possible to give more than 
one or two illustrations of the progress along each 
branch. ; 
Let us first take up the most interesting and im- 
portant question of the ‘‘characteristic’’ X-rays. It 
is known that every substance when bombarded by 
electrons of sufficiently high velocity emits X-rays of 
a quality characteristic of the substance. The interest 
of this comparison lies in the fact that it displays 
the most fundamental properties of the atom. The 
rays which each atom emits are characteristic of its 
very innermost structure. The physical conditions 
of the atoms ot a substance and their chemical asso- 
ciations are largely matters of the exterior; but the 
X-rays come from the interior of the atom and give 
us information of an intimate kind. What we find is 
marked by all the simplicity we should expect to be 
associated with something so fundamental. 
All the substances of atomic weight between about 
30 and 120 give two strongly defined ‘lines’; that 
is to say, there are found among the general hetero- 
geneous radiation two intense, almost homogeneous, 
sets of waves. For instance, rhodium gives two 
pencils of wave-lengths approximately equal to 
0-61 x 10-* cm. and 0-54 x 10-® cm. respectively. More 
exactly the former of these is a close doubtlet having 
wave-lengths o619x10-* ‘and o614x10~-§. The 
wave-lengths of palladium are nearly 0-58x10~-* and 
0-51 x10—*; nickel, 1-66 10-* and 1-50x 107°. Lately 
Moseley has made a comparative study of the spectra 
of the great majority of the known elements, and has 
shown that the two-line spectrum is characteristic 
of all the substances the atomic weights of which 
range from that of aluminum, 27, to that of silver, 
108. These X-rays constitute, there is no doubt what- 
ever, the characteristic rays which Barkla long ago 
showed to be emitted by this series of substances. 
