JANUARY 3, 1907] 
NATURE 22 
A 
ro) 
LEDRTERS TO RHE EDITOR. 
[The Editor does not hold himself responsible for opinions 
expressed by his correspondents. Neither can he undertake 
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No notice is taken of anonymous communications.] 
Radium and its Disintegration Products. 
Ix Nature of December 6, 1906, Mr. H. S. Allen has 
suggested that the difficulty encountered in introducing 
actinium with its four a-ray products between uranium and 
radium can be removed by assuming that the a particle 
is one-half of the helium atom, and he has applied this 
suggestion in a table showing six a-ray changes between 
uranium and radium. There would appear to be two 
serious and insurmountable objections to this view, how- 
ever, viz., (1) the continuation of the same line of reason- 
ing would lead to the assumption of no less than seven 
a-ray changes between radium and its final disintegration 
product, lead, while but four are known; and (2) the 
activity of the actinium in equilibrium with radium in 
minerals is entirely too low to permit any such conclusion. 
That lead is the final disintegration product of uranium 
is, I believe, conclusively shown by the fact that in un- 
altered primary minerals from the same locality the amount 
of lead is proportional to the amount of uranium in the 
mineral, and that in unaltered primary minerals from 
different localities the amount of lead relative to uranium 
is greatest in the minerals from the locality which, on the 
basis of geological data, is the oldest. 
In the case of a non-emanating, radio-active mineral 
containing no thorium, in which there is reason for 
assuming that the elements of the uranium-radium series 
have reached a state of equilibrium, the activity of the 
mineral in extremely thin films measured in an electro- 
scope with a large ionisation chamber is about 5-3 times 
as great as the activity of the uranium present in the 
mineral. The activity of the radium itself is about 0-52 of 
the activity of the uranium, and the activity of the radium 
products of rapid change together about 2-4 times that of 
the uranium. The activity of the radium F (polonium) is 
probably about 0-55 uranium, and is certainly not less 
than 0-5. The combined activity of the uranium, the 
radium, and the radium products is therefore about 4-5 
times the activity of the uranium alone. This leaves an 
activity of only o-8 that of the uranium which can be 
attributed to the activity of the four a-ray products of 
actinium. It was the knowledge of the approximate value 
of this factor which led Prof. Rutherford and the writer 
to conclude (Amer. Jour. Sci., xx., 56, 1905) that actinium 
was not a direct product of uranium in the same sense as 
is radium. 
The ranges of the four @ particles expelled by the 
actinium products have been determined by Hahn, and the 
average range of the four is 5-6 cm. The range of the 
a particle from radium itself is 3-5 cm. according to Bragg 
and Kleeman. If the particles are similar we would 
expect that the average particle from the actinium products 
would produce about 1-6 times the ionisation of the particle 
from radium. Since the activity of radium itself is 0-52 
times that of the uranium in the mineral, the activity of 
the four actinium products might be expected to be 
0-52 X 1-6X%4=3-32 uranium. The number actually found, 
as has been stated above, is only o-8 uranium, or one- 
fourth of this number. 
It will be noted in the above that the activity of the 
uranium is about twice that of the radium present, which 
is in good agreement with the conclusion of Moore and 
Schlundt that there are two a-ray changes in uranium, if 
it is assumed that the average range of the two uranium 
particles is about 3-5 cm. 
Although speculations of this sort are of doubtful value, 
the following suggestion may be sufficiently interesting to 
warrant its intrusion :—if the two changes in uranium and 
the five changes in radium are each assumed to take place 
with the expulsion of four a particles, and the four changes 
in actinium with the expulsion of only one a@ particle each, 
the conditions required by the relative activities of the 
various substances would appear to be fulfilled, and if, 
NO. 1940, VOL. 75 | 
moreover, the mass of each a particle be taken as 1, then 
the indicated atomic weights of the successive elements 
are in fairly good agreement with the accepted values. 
We have then uranium=238-5, actinium=230:5, radium= 
226-5, and radium F (lead)=206-5. In making this sug- 
gestion I fully appreciate that I am taking liberties with 
the accepted value of e/m for the a particle. 
It is of further interest to note that the activity of pure 
radium, calculated from the relative activity of the uranium 
and radium in minerals and the relative quantities present 
(Rutherford and Boltwood, Amer. Jour. Sci., xxii., 1), is 
indicated as about 1-4 10° times that of uranium, and the 
activity of pure radium bromide containing the equilibrium 
amounts of emanation and products of rapid change as 
about 3 X 10° times uranium. 
Bertram B. BoL_twoop. 
Sloane Laboratory of Yale College, New Haven, 
Conn., December 17, 1906. 
The a Rays. 
Tue a rays from radium appear to start life without 
electric charge, and subsequently become charged owing 
to collisions with the gas molecules they strike in their 
path. It seems, therefore, worth while inquiring what 
their behaviour would be if they were liable to become 
discharged again at a later collision, and to go on repeating 
this cycle during the ionising portion of their path. Very. 
possibly the @ particle is capable of losing more than one 
electron, in which case it would seem certain that it will 
have a greater charge at some portions of its path than 
at others. Looked at in this way the problem is a 
statistical one of considerable complexity, but my point of 
view will be sufficiently well illustrated by considering the 
average a particle to behave as if it had the following 
constitution. For a distance x of-its path it possesses an 
electric charge e. This is succeeded by a distance x’, 
during which its electric charge is e’. This is followed 
by a distance x with charge e, then a distance x’ with 
charge e’, and so on, repeating indefinitely. Let the 
particle have a mass m and initial velocity v,, then, con- 
fining our attention to a portion of the path so small that 
v, is not appreciably diminished by the collisions which 
occur, it is easy to show that the quantity measured by 
the electrostatic deflection as mv,*/e would really be 
INES), whilst the quantity measured by the electro- 
EXE X 
mie)(% + x") 
magnetic deflection as mv,/e would be SIE Thus 
CX FEX 
the measurements would give v, correctly, but the 
: exte'x’ E . 
quantity denoted by e/m would be FC aSayS It is evident 
that the apparent value of e/m would be independent of 
the pressure at which the measurements were made, since 
change of pressure changes both x and x’ in the same 
ratio. 
It is interesting to see what would happen if the a par- 
ticle were uncharged during one series of portions of its 
path, and carried the ordinary electrolytic unit of charge 
e during the alternating portions. If the alternate stretches 
were equal, this is what would be obtained if it were an 
even chance whether the a particle escaped with or with- 
out a charge after each encounter. In this case we should 
have x=x! and e’=o, and the measured e/m would really 
be e/2m. On this view Rutherford’s measurements would 
indicate that the a particles are hydrogen atoms with the 
normal charge instead of helium atoms with twice that 
charge. 
It may well be that it is a matter of chance whether 
the atom struck or the a particle retains the positive charge 
after an ionising encounter, but I do not wish to imply 
that this warrants the conclusion that the @ particle is a 
hydrogen atom. If we accept this conclusion we find our- 
selves face to face with serious difficulty in finding a place 
for helium in the story of radio-active change; but even if 
the a particle turns out to be a helium atom it is possible 
that its charge might vary periodically in something like 
the manner indicated. In this case the average charge 
would have to be twice the electrolytic unit. 
This kind of view has the advantage of affording a 
