: 
ec ial I 923] 
NATURE 
79 

Letters to the Editor. 
[The Editor does not hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he und-rtake to return, nor to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of NATURE. No notice is 
taken of anonymous communications.| 
On the Missing Element of Atomic Number 72. 
Since Moseley’s discovery of the fundamental laws 
of the X-ray emission, it has become quite clear that 
the most simple and conclusive characteristic of a 
chemical element is given oy its X-ray spectrum. In 
addition, Moseley’s laws allow us to calculate very 
accurately the wave-lengths of the X-ray spectral lines 
for any element in the periodic table, if those of the 
elements in its neighbourhood are known. Taking 
into account that the presence of a very small pro- 
portion of a definite element in any chemical substance 
suffices to give a good X-ray spectrum of this element, 
it is quite evident that for the eventual discovery of 
any unknown element X-ray spectroscopy, especially 
as it has been ite by Siegbahn, represents the 
most effective method. 
In the Comptes rendus of the Paris Academy of 
Sciences for May 22, 1922, Dauvillier announced the 
detection by means of X-ray spectroscopy of the 
element 72 in a mixture of rare-earth metals. This 
element was identified by Urbain with a rare-earth 
element, which he called celtium, the presence of 
which he had previously suspected in the same 
le. For different reasons, however, we think 
that Dauvillier's and Urbain’s conclusions are not 
justified. It appears from Dauvillier’s paper that at 
any rate the quantity of the element 72 in the sample, 
_ if present, must have been so small that it seems very 
bable that the element 72 should be identical 
with the element which in former papers Urbain 
claims to have detected in the same sample by in- 
vestigation of the optical spectrum and of the magnetic 
ies. The only lines which Dauvillier claims to 
ve detected are the lines La, and L§,, both of 
which he finds to be extremely faint (extrémement 
ible). The wave-lengths he gives, however, for these 
es are about 4 X.u. (rt X.u.=1o0-™ cm.) smaller 
than those which are obtained by a rational interpola- 
_ tion in the wave-lengths tables of Hjalmar and Coster, 
_ for the elements in the neighbourhood of 72. 
__ his theory of atomic structure. 
- 
ry 
From a theoretical point of view it appears very 
doubtful that the element 72 should be a rare-earth. 
It was announced in 1895 by Julius Thomsen from 
gee that from general consideration of the 
laws of the periodic system we must expect between 
tantalum, which in many compounds possesses 5 
valencies, and the trivalent rare-earths, a tetravalent 
element homologous to zirconium. The same view has 
also recently been put. forward by Bury on the basis 
of chemical considerations, and by Bohr on the basis of 
It is one of the most 
striking results of the latter theory, that a rational 
os Stiga of the appearance of the rare-earth 
me in the periodic system could be given. For 
these elements, according to Bohr, we witness the 
gradual development of the group of 4-quantum 
electrons from a group containing 18 electrons into 
a group of 32 electrons, the numbers of electrons in 
the groups of 5- and 6-quantum electrons remaining 
ed. Bohr was able to conclude that in the 
element lutecium (71) the group of 4-quantum electrons 
is complete, and we consequently must expect that in 
the neutral atom of the next element (72) the number 
NO. 2777, VOL. 111] 
of electrons moving in 5- and 6-quantum orbits must 
exceed that in the rare-earths by one. The element 
72 can therefore not be a rare-earth but must be an 
homologue of zirconium. 
In view of the great theoretical importance of the 
question we have tried to settle it by an experimental 
investigation of the X-ray spectrum of extractions of 
zirconium minerals. We have succeeded in detecting 
six lines which must be ascribed to the element 72 
(in Siegbahn’s notation La,, a, §;, Bz, 83, and 7). 
The complication was met that the lines La, and a, 
lie almost exactly in the place corresponding in the 
spectrum to the zirconium Ka,, and ag, lines in the 
second order. Difficulties which might arise from this 
fact may easily be avoided by keeping the tension on 
the tube between the critical tension of the zirconium 
K-lines (18,000 volts) and that of the L-lines of the 
missing element (10,000 volts). Besides, the relative 
intensity of the Ka lines is so different from that of the 
two La lines that any ambiguity is already thereby 
excluded. Not only the La lines but also the lines 
L§,, 8,, and 8, were, as regards their mutual distance 
and their relative intensity, in exact agreement with 
the expectation. The values which we obtained for 
the wave-lengths of the six mentioned lines all agree 
within one X.u. with those found by a Sg es 
Between our values for the lines La, and », and 
those published by Dauvillier, however, there exists 
the discrepancy referred to of about 4 X.u. (in general 
for other elements which have been measured by 
Dauvillier and by Coster the discrepancy is never 
more than 2 X.u.). Exposures under different 
conditions as well as a thorough discussion of the 
plates showed that the new lines found during our 
investigation cannot be ascribed to the first or higher 
order spectrum of any other known element. Our 
provisional results are: La, =1565°5; a,=1576; 
B, =1371'4; Bs =1323°7; Bs =1350°2; Yy=1177 X.U. 
More accurate and complete data as well as photo- 
graphs of the spectrum will soon be published. 
In a Norwegian zirconium mineral the new lines 
were so intense that we estimate the quantity of the 
element 72 present in it to be at least equal to one 
per cent. Besides we investigated with low tension 
on the tube a sample of “ pure zirconiumoxyde.” 
Also with this specimen the La lines were found, but 
very faint. It seems to be very probable that ordinary 
zirconium contains at least from o-or to o-1 per cent. 
of the new element. Especially the latter circum- 
stance proves that the element 72 is chemically 
homologous to zirconium. Experiments are in 
progress to isolate the new element and to determine 
its chemical properties. 
For the new element we propose the name Hafnium 
(Hafniae = Copenhagen). D. Coster. 
G. HEVEsy. 
Universitets Institut for teoretisk Fysik, 
Copenhagen, January 2. 

Continental Flotation and Drift. 
Tue theory that the continents have shifted their 
positions during geological time and, possibly, are 
still in motion has lately excited much discussion. 
The principal obstacle to its acceptance is the difficulty 
of adducing a force adequate to bring about the move- 
ments. Many years ago Osmond Fisher (‘‘ Physics 
of the Earth’s Crust’ p. 339) ascribed general con- 
tinental movements of this kind (accounting for 
the Atlantic rift, etc.) to the disturbance a the 
Pacific basin due to the genesis of the moon, on 
Darwin's well-known theory. Lately, Wegener has 
brought forward much evidence in favour of con- 
tinental movements. But I do not think~he has 
