

Marcu 4, 1915] 
NATEGRR fe 7 

a voltage of only about 13 volts, while about 80 volts 
are necessary to produce the lines of the 4686 series and 
the Pickering series. According to the theory of the 
writer, the energy necessary to remove the electron 
from the hydrogen atom corresponds to a fall of an 
electron through a potential difference of 13°6 volts, 
while the energy to be used in removing one electron 
from the helium atom corresponds to a fall of 29'0 
volts, and in removing both electrons to a fall of 
$3°4 volts. N. Bonr. 
Physical Laboratory, University of 
Mancheste:, February 21. 
X-Ray Fluorescence and the Quantum Theory. 
Tue experimental conclusions which I briefly out- 
lined in a letter to Narure of February 18 point 
directly to a theory of X-ray fluorescence and of the 
emission of radiation in quanta, which certainly bears 
a resemblance to Bohr’s theory of line spectra. The 
experimental evidence obtained is, however, so direct 
that there seems little possibility of escape from the 
conclusions given below. Indeed, the theory was 
forced upon the writer directly by the experimental 
- results, and it was only afterwards that he was re- 
minded of some similarity with the theory of Bohr 
based on the Rutherford atom. 
It is an experimental fact that in the case carefully 
investigated (and obviously in many, if not in all, 
other cases), the ejection from an atom of an electron 
associated with a fluorescent X-radiation of frequency 
mn necessitates an absorption of energy greater than 
the kinetic energy carried away by the electron by 
approximately the energy (hn) of one quantum of 
radiation of frequency n. Thus :— 
(1) Total absorption per electron emitted=3mv*+hn 
(approximately)—that is, the energy required to 
separate the electron (a K electron, say) from the parent 
atom, is approximately equal to the energy of a 
quantum of the fluorescent radiation of series K asso- 
ciated with that electron. 
The energy of a quantum of radiation may therefore 
be regarded as the mutual potential energy of the 
separated atom and electron, measured frcm the zero 
given by the electron in its normal position and state. 
When the displaced or any other electron falls backs 
into the position of the displaced electron, the energy 
is re-emitted as a radiation characteristic of the atom, 
and this, of course, in definite quantity. So much 
may be claimed as at any rate giving a first approxi- 
mation to the truth. The results of experiments, how- 
ever, suggest the possibility of the necessity for some 
modification of this theory in detail, though not in 
principle. For in the one case thoroughly investigated 
we get a nearer approximation to the experimental 
results by writing 
(2) Total absorption per electron emitted =3mv* + 
Tiny +h, where 7 and 7, are the frequencies of the 
kK and -L fluorescent radiations respectively. As the 
third term (hn,) is at its maximum value only about 
7 per cent. of the whole, it is impossible at this stage 
to say definitely whether or not it expresses a physical 
fact. This term was, however, suggested by a con- 
sideration of the probable process following the ejec- 
tion of a K electron. The relation indicates that 
possibly the energy required to free a K electron is 
equal to the sum of the energies of quanta of K, L, 
and any other fluorescent radiation of lower series M, 
N, ete.—presumably originating in vibrations in the 
outer rings of the atom. 
If we accept this provisionally it means that the 
energy of a quantum of K radiation is that required 
to displace a K electron into the position of an L elec- 
tron, while the energy of a quantum of L radiation 
NO. 2366, VOL. 95| 

is that required to displace an L electron into the posi- 
tion of an M electron, and so on. Such a process 
may never occur; it is, however, a convenient way of 
expressing the energy required completely to eject the 
electron in terms of steps which can only be regarded 
as extremely probable in the inverse process involving 
radiation. 
Thus the energy of a quantum of K radiation is left 
in the atom from which a K electron is hurled; or 
possibly the energies of one quantum of each of the 
fluorescent radiations, K, L, M, etc., are left in the 
atom. This energy must, of course, be radiated while 
the atom is regaining its original configuration by the 
absorption of an electron into the K position. It 
seems probable, however, that the readjustment of the 
atom and the principal radiation take place even before 
the atom as a whole regains an electron, by an L 
electron falling into the position of a displaced K 
electron, an M electron replacing an L electron, and 
so on; only the final stage of the readjustment being 
completed by the absorption of an electron into an 
outer depleted ring. 
It is obvious in this case—unlike that studied by 
Bohr—when and why an electron falls into an inner 
ring; it is simply subsequent to and due to the re- 
moval of an inner ring electron. No new principle of 
radiation is involved, yet it accounts for radiation 
taking place in quanta. We should thus expect L 
radiation to be associated with the emission of K 
electrons as well as with L electrons. Search for 
such a radiation is at present being made. Pointing 
to the probability of such an associated radiation is 
the fact that when hn, becomes a smaller fraction of 
the whole absorption, the discrepancy found when it 
is omitted, as in equation (1), diminishes. Not only 
is this so, but the energy of the corpuscular radiation 
and of the K fluorescent radiation actually emitted, do 
not guite fully account for the whole energy absorbed. 
The discovery of the L radiation in calculated intensity 
would give almost perfect agreement. 
In spite of these indications the writer hesitates to 
make a definite statement about the physical reality of 
the third term concerned with L radiation; experi- 
ments will very soon decide the point. 
In either case we have the direct evidence that the 
energy of a quantum is simply energy absorbed in 
removing the corresponding electron from its normal 
orbit; it is the energy afterwards set free, presumably 
when the electron returns. 
It is hoped that experiments now being undertaken 
will determine also if X-ray fluorescence—that arising 
from the vibration of inner ring electrons—can be 
appreciably delayed by retarding the return of the 
ejected or other electrons from outside the atom. It 
is more probable that X-ray phosphorescence will not 
be detected, the readjustment of the interior of the 
atom taking place immediately after the ejection of 
an inner electron, and the final absorption of an elec- 
tron into a surface ring being the only part of the 
process susceptible to external conditions. The subject 
can, however, only receive adequate treatment in com- 
munications to other journals. C. G. Barkra. 
University of Edinburgh, February 27. 
The Physical Properties of Isotopes. 
Pror. Soppy’s letter in Nature of February 4 would 
seem to lead to certain interesting conclusions about 
the structure of the atom. It is easy to show that 
| two elements of different atomic weight must differ 
either in their chemical or in their physical properties. 
If elements are inseparable chemically their affinity A 
ry 
must be equal. Now A= -r{ wan if T is the tem- 
° 
