706 
NATURE. 
[| May. 26, 1923 

aluminium compound, which does not, however, 
occur in the list of substances given by Prof. Lewis. 
In resuming, during July last, a series of spectro- 
scopic investigations begun in 1913 on the lines of 
earlier work by the present Lord Rayleigh and Prof. 
Fowler, I attempted a preliminary. observation of 
the spectrum resulting from the introduction of the 
vapour of aluminium chloride into the stream of 
active nitrogen. After a long -exposure a solid 
deposit was produced on the inside of the afterglow 
tube, and when the stream of active nitrogen was 
passed through the same tube a few days later the 
deposit exhibited a bright green fluorescence. It 
was hoped that this observation might be recorded 
in a future paper, after opportunity for further 
spectroscopic work had occurred. In view of Prof. 
Lewis's announcement, however, perhaps this note by 
way of corroboration is not out of place. 
W. JEvons. 
Physics Department, 
Artillery College, 
° Woolwich, May rr. 
The Dissolution of the Conjoint Board of Scientific 
Societies. 
AcTING under the instructions of the Executive 
Committee, we have now wound up the affairs of the 
Conjoint Board of Scientific Societies. 
Everything connected with the “‘ List of Scientific 
Periodicals ’’ has been placed in the hands of the 
Trustees who have been appointed to carry out this 
publication. The work on the List is now well 
advanced. 
The Royal Society has agreed to accept the custody 
of filed records and documents related to the work of 
the Board and its Committees, with the exception of 
that just mentioned. These have now been lodged 
with the Society, except the records of three Com- 
mittees, at present in the hands of their secretaries 
for final revision before lodgment with the Society. 
Copies of the proceedings of the Board have also 
been deposited with the British Museum, the Royal 
Society of Edinburgh, the Patent Office, Sir Arthur 
Schuster, Sir Herbert Jackson, and Prof. W. W. 
Watts. Sets of printed matter have been also handed 
to the Department of Scientific and Industrial 
Research, the National Physical Laboratory, the 
Science Library, the University of London, the 
Imperial College of Science and Technology, the 
British Museum (Natural History), and the London 
Library. HERBERT JACKSON. 
W. W. Watts. 
May 14. 
The Capture of Electrons by Swiftly Moving 
Alpha Particles. 
Tue swiftly moving a- particle produces a large 
number of ions in its passage through a gas, as is 
evidenced by the beautiful Wilson photographs of 
a-ray tracks. It is a matter of some surprise that 
at the act of ionising a molecule, or immediately 
after, the a-particle does not attach one or more 
of the free electrons to itself. The approach is very 
near and the force is that due to a double charge. 
The experiments of Rutherford, Marsden and 
Taylor, and others indicate that the swiftly moving 
a-particles begin to take up electrons when the velocity 
has decreased to 0-4 of the initial value (v,=0-4 v,). 
The recent experiments of Henderson (Proc. Roy. Soc., 
January I, 1923) indicate that at this velocity the 
a-particle takes up one electron. It then continues 
NO. 2795, VOL. 111] 

its smashing career through matter without change 
until the velocity is reduced to 0-15 of the initial 
value (v,=0'15U»)). At this velocity, approximately, 
the a-particle takes up a second electron and becomes 
a more or less harmless atom of helium. 
The limiting velocities at which the a- particle 
captures the first and then the Second electron seem 
to be rather definitely fixed. The initial velocity 
from radium-C is v)=2-06 x 10° cm./sec. The velocity 
at first capture is v,=0-4v,=8-2 x 108 cm./sec. The 
velocity at second capture is stated by Henderson to 
be at least as small as v,=0-15v)9=3'1 x 10° cm./sec. 
It is desired to point out here that this failure to 
capture an electron may be due to the high velocity 
of the a-particles. The free electron will at once 
start toward the a-particle. If the latter is moving 
with a velocity greater than the velocity of fall 
(parabolic velocity) of an electron into the K ring, 
the electron will fail to reach the K ring and effect 
a combination. Having this situation in view, I 
have calculated the limiting parabolic velocity for 
an electron falling into the K ring of (1) a doubly 
charged a-particle and (2) one having a single charge. 
The radius of the K ring is given by a=h*/47*meE, 
where E is the excess nuclear charge in each case. 
This velocity is given by }mv*=Ee/a. From these 
considerations the calculated velocity for the first 
case is v,=6:2x10% cm./sec. In the second case 
the charge E is single, and the velocity v,=3-2 x 10° 
cm./sec. The experiments are necessarily not very 
exact, but the agreement is sufficiently close to 
suggest that this may be the proper explanation of 
the action. 
From this point of view, all a-rays, of whatever 
initial velocity, should capture the first and second 
electrons at the same velocity. This is a matter 
of sufficient importance to determine experimentally. 
BERGEN Davis. 
Columbia University, 
New York. 
Recent Aurore. 
MaGnetic disturbances and associated phenomena 
have perhaps a special interest when they occur 
during the minimum period of sunspot activity, 
owing to the comparative rarity of these events at 
this time. When, therefore, I read in Nature of 
April 21, p. 534, the account by Father Cortie of the 
recent disturbances, it reminded me of observations I 
had made of the aurora in activity on the dates 
referred to. 
with a nine-days-old moon shining, the northern 
horizon was seen to be brightly illuminated by auroral 
light at 8.45 p.m. for about half an hour, but no 
streamers were seen. On March 24, again in first- 
quarter moonlight, but a very hazy sky, I saw an 
auroral display of unusual beauty at 9 P.M. over 
Bassenthwaite Lake. The arch was elevated ten 
degrees, with streamers and lances shooting upwards 
for, in some cases, another thirty degrees. The 
length of the arch I could not measure, owing to each 
end being hidden by lofty mountains, but it was 
visible for sixty degrees. It was clear-cut below, and 
merged gradually into the moonlit haze above. 
The reflection of the streamers in the perfect 
mirror-like surface of the lake, and the shining arch 
flanked by snow-capped heights, dimly seen in the 
misty moonlight, combined to form a picture of 
indescribable beauty. 
W. B. Housman, 
Seaton, Cumberland, 
April 25. 
On February 25, in a very clear sky, _ 

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