498 
NATURE 
[JULY I, 1915 

to the fact that these high-speed electrons arise from 
one or more of the observed frequencies in the y-ray 
spectrum. 
In order to account for such results, it seems neces- 
sary to suppose that the y rays of high frequency are 
not necessarily emitted as single pulses, but consist 
of a train of pulses either produced simultaneously or 
following one another at very short intervals. Each 
of these pulses has an energy hv corresponding to the 
frequency v, but the total energy in the train of waves 
is phv where p is a whole number, which may have 
possible integral values 0, 1, 2, 3, . . . efc., depend- 
ing on the structure of the atom and the conditions of 
excitation. The penetrating power of such a train of 
waves corresponds to that of a single wave of fre- 
quency v, but on passing through matter the energy of 
the whole train of p waves occasionally may be trans- 
ferred to an electron which consequently is expelled 
with an energy phv. There is very strong evidence of 
the general correctness of this point of view, for most of 
the stronger lines in the $-ray spectrum of radium C 
have energies which correspond to an integral multiple 
of the energy corresponding to the strong lines actually 
observed in the y-ray spectrum It seems probable 
that under the ordinary conditions of excitation by 
kathode rays in a vacuum tube, the X-ray contains 
only one pulse or wave, but under the far more 
powerful stimulus of the very swift 8 particle escaping 
from the atom, a long train of waves, each of the 
same frequency, is produced. The energy of the whole 
train of waves may under suitable conditions be given 
to an electron, which consequently has a speed very 
much greater than that impressed upon it by a single 
wave of the same frequency. 
Limit to the Frequency of Vibration of the Atom. 
There is one question of fundamental importance 
which arises in considering the modes of vibration of 
the atom, viz. whether there is a definite limit to 
the frequency of the radiation which can be excited in 
a given atom. Theory does not provide us with an 
answer to this problem, since little is known about 
the conditions of excitation, nor even of the nature 
of such high-frequency vibrations. A study of the 
frequency of the y rays from radio-active substances is 
of great importance, as it throws much light on this 
problem. 
As we have seen, the energy of the 8 particle escap- 
ing from the nucleus of radium C is equivalent to that 
acquired by an electron moving in an exhausted space 
under a potential difference of several million volts. 
This high-speed electron passes through the electronic 
distribution in its escape from the atom. Notwith- 
standing such ideal conditions for the excitation of 
high-frequency radiations of the atom, the highest 
frequency in the radiation emitted by radium C is only 
about twice that obtainable from an ordinary hard 
X-ray tube excited by 100,000 volts. It thus appears 
probable that there is a definite limit to the frequency 
of the radiation obtainable from a given atom, however 
high the speed of the disturbing electron. This limit- 
ing frequency is determined not by the speed of the 
electron but by the actual structure of the atom. Since 
the y radiation from radium C gives a line spectrum. 
it would appear that the highest frequency obtainable 
is due to a definite system of electrons which is set 
into characteristic vibration by the escape of a £ par- 
ticle. In order to throw further light on this point, 
Prof. Barnes, Mr. H. Richardson and myself have 
recently made experiments to determine the maximum 
frequency obtainable from an X-ray tube for different 
constant voltages. The Coolidge tube, which has 
recently been put on the market, is ideal for this pur- 
pose, as it provides powerful radiation at any desired 
voltage. The anti-kathode is of tungsten of atomic 
NO. 2383, VOL. 95| 


weight 184, so that we are dealing in this case with 
the possible modes of vibration of a heavy atom. The 
maximum frequency of the radiation was deduced by 
measuring the absorption by aluminium of the most 
penetrating rays emitted at different voltages. The 
absorption of X-rays of different frequencies by 
aluminium has been examined over a very wide range, 
and can be expressed by simple formula. It was 
found that for 20,000 volts the frequency of 
the radiation was slightly lower than that to 
be expected if Planck’s relation held. With 
increasing voltage there is a rapid departure 
from Planck’s relation. The frequency reaches a 
maximum at about 145,000 volts, and no increase was 
observable up to the maximum voltage employed, 
viz. 175,000 volts. The experiments thus show that 
the frequency of radiation reaches a definite maximum, 
which is no doubt dependent on the atomic weight of 
the particular radiator employed. It is of interest to 
note that the maximum penetrating power of the 
X-rays from the Coolidge tube in aluminium is about 
the same as the y rays from radium _ B, 
but is about 3/10 of the y rays from radium 
C. There is evidence which suggests that the 
very penetrating y rays from radium C correspond to 
the octave of the ‘IX’ characteristic radiation of that 
element. If this be the case, it may prove possible 
that a still more penetrating radiation might be ob- 
tained from tungsten, but in order to excite it a volt- 
age of the order of a million volts would probably 
be required. In any case, it seems clear that Planck’s 
relation does not hold for excitation of high fre- 
quencies by swift electrons, but may hold very approxi- 
mately for lower frequencies corresponding to the 
radiation excited by a few hundreds or thousands of 
volts. On the other hand, the evidence obtained from 
a study of the 8 rays excited by X-rays or y rays 
certainly indicates that the relation E=phv holds at 
any rate very approximately for the highest frequency 
examined. It is thus obvious that the emission of 
B and y rays from the radio-active atoms is clearly 
connected with the general theory of radiation, and 
it seems likely that a close study of these radiations 
will throw much light on the mechanism of radiation 
in general. 
There can be little doubt that the penetrating 
y rays from active matter have their origin in the 
vibration of electronic systems in the structure of the 
atom outside the nucleus. The nucleus itself, how- 
ever, must be violently disturbed by the expulsion of 
an a or #8 particle. If this leads to the emission of 
a y radiation, it must be of exceedingly high fre- 
quency, as the forces holding together the component 
parts of the nucleus must be exceedingly intense. We 
should anticipate that this radiation would be extra- 
ordinarily penetrating, and difficult to detect by elec- 
trical methods. So far no experimental evidence has 
been obtained of the existence of such very high 
frequency radiations, but it may be necessary to devise 
special methods before we can hope to do so. 

UNIVERSITY AND EDUCATIONAL 
INTELLIGENCE. 
CAMBRIDGE.—Reference is made in the Times of 
June 25 to the large number of Cambridge men now 
on active service; from a ‘‘ War List”? which has just 
been issued it appears that 8885 members of the 
University are with the colours. The official list gives 
the names of those who have been killed or wounded 
in action, and it appears that for every four who have 
been wounded three have been killed. This is due to 
the fact that most of the Cambridge men serving are 
either 1st or 2nd lieutenants. 
