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spectrum due to £ rays of all possible velocities, on 
which is superimposed a line spectrum due to a small 
number of # particles of definite velocity comprising 
‘each group. 
Lines in the f-ray spectrum have been observed 
for particles which have a velocity not far from that 
of light, but the photographic effect of the particles 
‘becomes relatively feeble for such high speeds. 
It is known from direct measurement that each 
atom of radium B or of radium C in its disintegration 
emits on an average one B particle. In the B-ray 
spectrum of radium C at least fifty definite bands are 
observed, differing widely in intensity. It is thus 
clear that a single atom in disintegrating cannot 
yprovide one B particle for each of these numerous 
groups. It is thus necessary to conclude that each 
atom does not emit an identical 8 radiation. The 
‘results are best explained by supposing that the B-ray 
spectrum is the statistical effect due to a large number 
of atoms, each of which may only give one or two 
of the groups in its disintegration. In this respect a 
B-ray transformation is distinguished from an a-ray 
‘transformation, for in the latter case each atom emits 
one a particle of characteristic speed. It will be seen 
‘later that there is undoubtedly a very close connection 
‘between the emission of 8 and y rays from radio-active 
atoms, and the probable explanation of the remark- 
ably complex £-ray spectrum will be discussed later. 
With the exception of one element, radium E, and 
possibly uranium X, all the radio-active substances 
which emit primary 8 rays give a line spectrum. For 
‘the majority of elements the strong lines in the B-ray 
Spectrum have been determined by Baeyer, Hahn, and 
Meitner, but more intense sources of radiation will 
‘be necessary to map accurately the weaker lines. 
y Rays. 
The earlier experiments on the y rays were mainly 
‘confined to a determination of the absorption of the 
more penetrating radiations by different kinds of 
matter. It was early observed, however, that some 
‘of the radiations appeared to be complex. This was 
shown by anomalies in the initial part of the absorp- 
tion curve. In the meantime, a notable advance in 
our knowledge of X-rays had been made by the work 
of Barkla. He found that under certain conditions 
‘each element when bombarded by X-rays of suitable 
penetrating power gave rise to a strong radiation 
which was characteristic for that element, e.g., the 
lighter elements from aluminium to silver emitted 
characteristic radiations called the “‘“K” series, which 
‘increased rapidly in penetrating power with the atomic 
weight of the radiator. It was found that the heavier 
elements emitted in addition another characteristic 
radiation of softer type, which was called the “L” 
series. These results showed clearly that there must 
‘be definite structures within the atom which gave rise 
to a definite radiation under suitable conditions of 
excitation. From these results it seemed probable 
that the y rays from radio-active matter must consist 
of the characteristic radiations of these heavy 
‘elements, analogous in type to the corresponding 
radiations observed in ordinary elements when 
excited by X-rays or kathode rays. These conclusions 
were confirmed by a series of investigations made by 
Rutherford and Richardson. The y rays were analysed 
‘by means of their absorption by aluminium and by 
lead, the disturbing effects of the primary 8 rays 
‘being eliminated by means of a strong magnetic 
field. It was found, for example, that the y rays from 
radium B, when examined by their absorption in 
aluminium, consisted of at least two types, one easily 
absorbed, and the other eighty times more penetrat- 
‘ing. By further observations of the absorption of the 
NO. 2383, VOL. 95] 
NATURE 

[JuLy 1, 1955 

y rays by lead, Richardson found that the rays from 
radium B could be divided into at least four definite 
types, each of which was absorbed exponentially by 
lead. Similar results were obtained for all the radio- 
active elements which emitted y rays. In some cases 
the soft y rays, e.g., those from radium B, corre- 
sponded to the characteristic radiation of the ‘‘L” 
series, and others to the ‘‘K” series. The general 
results, however, indicated that several additional 
series of characteristic radiations are present in some 
cases. It was clear from these experiments that the 
y rays corresponded to the natural modes of vibration 
of the inner structure of the radio-active atoms. In 
the meantime the experiments of W. H. Bragg and 
W. L. Bragg, and of Moseley and Darwin, had shown 
that the characteristic X-radiations of the elements 
gave definite and well-marked line spectra. These 
spectra were simply determined by reflecting the rays 
from crystals. If this were the case, it seemed prob- 
able that the y rays from the radio-active atoms would 
also give line spectra, and thus allow the natural 
frequencies of vibration of these atoms to be deter- 
mined. During the past year, a number of experi- 
ments have been made to test this point by Ruther- 
ford and Andrade, using radium B and radium C as 
the source of y radiation. As was anticipated, it was 
found that the y rays from radium B and radium C 
gave well-marked line spectra. The general method 
employed was to use an a-ray tube containing a large 
quantity of emanation as a‘source of radiation. The 
y rays were reflected from a crystal of rock salt, and 
the position of the spectrum lines determined photo- 
graphically. Usually twenty-four hours were neces- 
sary to obtain a marked photographic effect. Special 
difficulties arose in these experiments which are absent 
in an investigation of a similar kind with X-rays. In 
addition to y rays, the radio-active matter emits very 
penetrating 8 rays which have a strong photographic 
action; while the y rays in their passage through 
matter themselves give rise to high-speed 8 rays. The 
disturbing effect of these radiations has to be elimin- 
ated by placing the whole apparatus between the poles 
of a powerful electromagnet. In this way it was 
found that the spectrum of radium B consisted of a 
large number of lines, of which the most intense were 
deflected at angles of 1° 46’, 10°, and 12°. The more 
penetrating radiation from radium C gave a strong 
line of 1° and a fainter line at 43’. The strong lines 
at 10° and 12° are due to easily absorbed y rays, and 
undoubtedly correspond to the “tL” radiation of 
radium B. The line at 1° corresponds to a very 
penetrating radiation which has a wave-length less 
than 1/1toth of an Angstrém unit. The penetrating 
y rays from radium C have by far the shortest wave- 
length so far observed. It does not seem probable that 
such short waves can be produced artificially in an 
X-ray tube unless possibly an exceedingly high voltage 
be applied. : 
There is one interesting result of these investiga- 
tions that should be mentioned. The two strong lines 
of the radium B spectrum deflected at 10° and 12° 
were found to correspond exactly in position to the 
X-ray spectrum of lead. These experiments thus con- 
firmed the view based on chemical evidence that 
radium B and lead were isotopic, i.e., they were 
elements of practically identical chemical and physical 
properties, although their atomic weight differed by 
seven units. 
Connection between B and y Rays. 
Before considering in detail the difficult problem 
of the connection between 8 and ¥ rays, it is desirable 
to summarise the main facts that have been estab- 
lished in regard to the relations between kathode rays 
and X-rays :— 

