JULY 9, 1914] 
as old as the oldest of those figured in the respective 
memoirs of M. Fraipont and Sir Ray Lankester, 
carries the usual pair of conical bony horn-cores, which 
appear to have been devoid of terminal antler-like 
caps. In place of these being covered with hairy skin, 
the specimen, as mounted by Messrs. Gerrard, shows, 
however, that they were invested with (so far as I was 
able to determine) true horny sheaths, resembling 
candle-extinguishers, and_ recalling the terminal 
sheaths surmounting the hair-covered horn-cores of a 
prongbuck with newly developing horns figured by Dr. 
Sclater on p. 540 of the Proc. Zool. Soc. for 1880. 
Messrs. Gerrard were positive that the sheaths came 
with the skin, and as they appear to correspond in 
size with the bony cores, I see no reason to doubt the 
statement, more especially as the sheaths cannot 
apparently have pertained to any adult antelope. 
Were it not for the fact that Dr. Christy is at pre- 
sent somewhere in the Belgian Congo, collecting on 
behalf of the Museum at Tervueren, I should have 
deferred making any statement on the subject until I 
had communicated with him. But as jit may be 
months before I get a reply to a letter just dispatched 
(even if it ever reaches its destination), 1 have con- 
sidered it advisable to put my observations on record, 
without, however, for the present, making them the 
basis of any deductions or speculations. 
R. LyDEKKER. 
Thorium Lead—An Unstable Product. 
Tue work of Boltwood and Holmes some years ago 
on the occurrence of lead and uranium in minerals 
rendered it very improbable that the end product of 
thorium could be lead. From recent generalisations, 
however, in respect to radio-elements and the periodic 
law, it is to be expected that the end products of the 
radio-active elements should all be isotopic with lead. 
One method of attacking the problem is the deter- 
mination of the atomic weight of lead extracted from 
uranium and thorium minerals. On the assumption 
that radium G and thorium E are stable, a know- 
ledge of the composition of the mineral from which 
the lead has been extracted enables one to calculate 
the expected value for the atomic weight of the lead. 
Comparison of this value with that found experiment- 
ally gives a means of testing whether radium G and 
thorium E are stable or not. 
Using this method, Soddy and Hyman (Trans. 
Chem. Soc., 1914, vol. cv., p. 1402) obtained a result 
for lead from a thorite rich in thorium and. poor in 
uranium, which indicates that thorium E is stable. 
On the other hand, Richards and Lembert made a 
determination on lead extracted from thorianite, which 
points to the instability of thorium E (see Fajans, 
Heidelberger Sits. Ber. A., 1914, Abh. 11). Holmes 
(Nature, April 2, 1914) came to a similar conclusion 
by an examination of the ratio Pb/U, for a series of 
analyses of radio-active minerais. 
stable, this ratio should be constant for minerals of 
the same geological age, but it should increase with 
the age of the mineral. 
fied. In order to examine, the question more fully, 
Holmes and the present writer examined the lead, 
uranium, and thorium contents of a series of radio- 
active minerals of Devonian age, from the same 
locality in Norway. The results of this investigation, 
shortly to be published, indicate very strongly that 
thorium E is unstable, and that it cannot therefore 
be regarded as the end product of thorium. 
The present letter indicates how the above results 
have been applied by the writer to determine the half 
period value of thorium E, and the method has the 
oapee3 2, VOLet Ogi 
If thorium E be |! 
Neither criterion was satis- | 
NATURE 
479 
advantage that it is quite independent of whether 
thorium lead (thorium E) is stable or not. A more 
detailed discussion of the question and its consequences 
will be published in the near future. 
Amongst the minerals analysed by Holmes and the 
writer were several thorites and orangites, rich in 
thorium, and well adapted for an examination of the 
question of the stability of thorium E. These minerals 
being all of the same age, the total lead present may 
be regarded as the sum of the following three con- 
stituents: (1) Original lead (Pb,), (2) uranium lead, 
(3) thorium lead. Further, whether uranium lead and 
thorium lead are stable or unstable, we can express the 
above statement as an equation thus :— 
Pb=Pb,+A.Th+«.U. 
Here Pb, U, and Th represent the content of the 
mineral in lead, uranium, and thorium respectively ; 
X\ is the amount of thorium E in equilibrium with 
1 gram of thorium, and « is the amount of uranium 
lead present in the mineral per gram of uranium. 
This last factor «x is constant for minerals of the same 
age, and varies in sympathy with the age of the 
| mineral—this indicating that radium G is a stable 
| product. 
The amount of original lead was assumed 
constant, since the minerals used were similar and 
from the same locality. Using the results of the 
analyses of three minerals, three equations are obtained 
by substitution in that above, and from these equations 
the values of A, x, and Pb, can‘be calculated. This 
calculation was performed with three different mineral 
combinations, and consistent results were obtained. 
The value of X found was 4x 10-° gram. The value of 
x found was 0-042, a result known to be correct from 
other considerations. Now it can readily be shown 
that the lead-producing power (calculated from the 
helium generation) of thorium is about 0-4 that of 
uranium. Whence, if thorium E is stable, the value 
of X should be 0-4x0-042=0-017. The low value 
(4x 10-°) actually obtained seems to prove beyond 
question that thorium lead is unstable, and that it has 
a half period equal to 4x 10o-° times that of thorium, 
or 4.10=5 x 1-5.10%°=6.10° years. It does. not seem 
likely that thorium lead (thorium E) emits a rays, for 
these should have a range of about 3 cm., and would 
have been detected. If, on the other hand, it emits 
B rays, it is to be expected that bismuth would prove 
to be the end product of thorium. In any case, the 
systematic examination of radio-active minerals for 
bismuth seems highly desirable, for if it is the stable 
end product of thorium, the ratio Bi/Th will be found 
constant for minerals of the same geological age, and 
this ratio will vary in sympathy with the age of the 
mineral. ‘Thus this ratio could be used for the deter- 
mination of geological time just as that of lead to 
uranium has hitherto been used by Holmes (‘* The 
Age of the Earth,’ London; 1913) for the same 
purpose. If the bismuth isotope from thorium is un- 
stable, the method indicated in this letter could be 
used to find its half period, and thus further informa- 
tion could be gathered as to the direction of the 
succeeding disintegration, i.e., whether an a ray 
change brings the end product into Group III.B 
(Thallium) or a f ray change carries it still further to 
the Polonium Group (VI.B). 
The one doubtful assumption in the present treat- 
ment is that in the minerals used for the calculation 
of X, the percentage of original lead present is the 
same. This assumption is not without foundation, 
and in a forthcoming publication the writer will adduce 
evidence in support of the assumption in the case of 
the minerals used. Ropert W. Lawson. 
Radium Institute, Vienna. 
