ApRrIL 7, 1923] 

























these elements. As mentioned in our previous letters, 
however, the two lines ascribed by Dauvillier to the 
element 72 were lying 4 X-units distant from our 
Hf-lines, which is distinctly more than the limit of 
_ experimental error,’ whereas the lines of the elements 
7° and 71 measured by Dauvillier (Comptes rendus, 
vol. 174, p. 1347, 1922) on the same plates closely 
=< with the measurements of the same elements 
obtained by Coster (Phil. Mag., vol. 44, p. 546, 1922). 
As the two lines according to Dauvillier were ex- 
tremely faint, they may easily be explained to be of 
some other origin. 
It is of interest to note that, at various times, 
“announcements have been made as to the complexity 
of zirconium. In 1845 Svanberg claimed that in 
decomposing zircons he discovered a new element 
“norium,” with a lower atomic weight than zircon- 
ium. His and Sjégren’s (1853) statements were later 
disproved by the work of several investigators in- 
cluding Marignac. In 1864 Nylander reported the 
existence of two earths in zirconia. Five years later, 
by a spectroscopic investigation of zirconium, Sorby 
was led to announce the discovery of “ jargonium ”’ 
and Church of “ nigrium.”’ Finally, in r901 Hofmann 
and Prandtl thought that they had found in euxenite 
a new element related to zirconium. It is also 
interesting to note that Mendeleéff, as we learn from 
‘Sir T. E. Thorpe’s letter in NarurE of February 24, 
'p. 252 (March 17), suggested that the extraordinarily 
discordant values for the atomic weight of titanium, 
found by several chemists, might be due to the 
presence of a homologous element of higher atomic 
weight in their material. Whether these statements 
in some cases may be explained by the presence of 
hafnium in the minerals and preparations under 
investigation, it is not easy to decide. The intricate 
chemistry of zirconium, and the great chemical 
similarity of hafnium with this element, would in fact 
have made any establishment of hafnium very 
difficult before the development of the powerful 
method of X-ray analysis. D. Coster. 
3 G. HEVEsy. 
Universitetets Institut for teoretisk Fysik, 
; Copenhagen, March 20. 

Constitution of Black Maketu Sand. 
__ Tue letter of Messrs. Smithells and Goucher in 
Nature of March 24, p. 397, under the above title 
calls for a short reply from me. 
_ The authors do not state with what object their 
) riments were made, but the results of these 
iffer so much from my own as to suggest that the 
sand examined by them was from another part of 
the deposit at Maketu, or possibly from an entirely 
different source, such as that on the Taranaki Coast. 
Is “ Prof. Bohr’s conclusion that no new element 
is present ’’ to be found in any paper published by 
him ? If not, it would be interesting to know upon 
what authority the authors quote. it. 
As my original communication on this subject was 
made to the Chemical Society, I feel it my duty to 
send to the Society, in the first place, the results of 
my own further experiments and also those of the 
examination of my preparations by Drs. Coster and 
Hevesy. This I hope to be able to do in time for 
publication in the Society’s Journal for April. 
F ALEXANDER SCOTT, 
34 Upper Hamilton Terrace, London, N.W.8, 
March 26. 
1 Dauvillier’s measurements carried out since the announcement of our 
_ discovery, on other material which possibly contained hafnium, have already 
led him to give new values for the same wave-lengths, which are respectively 
+34 and 2-3 X-units larger (NaTuRE, February 17, 1923) 
No. 2788, VoL. 111] 
NATURE 

eee 
463 

Tracks of a-Particles in Helium. 
In a recent issue of NaturE (January 27, p. 114), 
Messrs. Ryan and Harkins have published some 
photographs of the ionisation tracks of recoiling atoms 
produced by collision of a-particles with air mole- 
cules. We have been also engaged in photographing 
the tracks of a-particles from polonium in helium, 
and have obtained some interesting photographs. 
Besides the long range recoil helium atoms, we have 
obtained a few photographs in which are shown the 
ionisation tracks of all the constituent parts of a 
helium atom, namely, of the nucleus and the two 
bound electrons. They are shown in Fig. 1 (i and ii). 

Fic u 
It will be noticed that both the electrons are ejected 
on the same side of the a-particle track. One of 
us (D. Bose, Zeit. f. Phys., 12, 207, 1922) has pre- 
viously photographed the ionisation tracks of several 
thousands of a-particles in hydrogen, and in no case 
was a photograph obtained which showed simul- 
taneously the ionisation tracks of the two constituents 
of a hydrogen atom. This behaviour can well be 
explained on the model of the normal helium atom 
proposed by Lande and others, according to which 
the two electrons move in orbits which are inclined 
to one another. If an a-particle strikes the atom*at 
the moment when both the electrons are near the 
point where their orbits cross, then the probability 
of their both being ejected in the same direction is 
very great. 
The photograph (Fig. 1, iii, a) presents some special 
points of interest. In it is shown (1) the track of the 
a-particle before and after collision, (2) that of a 
recoiling nucleus, (3) four small tracks which radiate 
out from the circular patch and are due to the electrons 
which are ejected from the atom under collision. 
The circular patch, which is absent in the photographs 
of other recoiling atoms, resolves when seen under 
high magnification into a number of lines radiating 
from a centre. An enlarged photograph which has 
been slightly retouched is reproduced in (iii, b). It 
will be noticed also that the path of the recoil atom 
is very much curved in the beginning—a phenomenon 
we have not observed before in the many tracks of the 
recoil atoms which we have obtained in air, hydrogen, 
and helium. Its length is 4 cm., and is larger than 
any of the other recoiling atom tracks which we have 
obtained in helium. 
Judging from the number of the electrons which 
have been emitted we have here evidently the collision 
of an a-particle with an atom more complex than 
either hydrogen or helium; we can suppose it to be 
either nitrogen or oxygen; such atoms can well be 
expected to be present as impurities. But it is difficult 
