A \VEBKLY ILLUSTRATED -JOURNAE; OF, SCIENCE: 
‘“ To the solid ground 
Of Nature trusts the mind which builds for aye.’’—WoRDSWORTH. 
THURSDAY; «MARCH. . 5; 
1914. 
RADIO-ACTIVE ELEMENTS AND THE 
PERIODIC TABLE. 
The Chemistry of the Radio-Elements. Part ii. 
The Radio-Elements and the Periodic Law. By 
F. Soddy. Pp. v+46. (London: Longmans, 
Geeen and-Co. , 1914+); Pricé ‘2s:) net. 
R. SODDY’S contention that there exist 
sets of elements, incapable of separation 
from each other by chemical means, has much 
experimental evidence to support it, taken from 
the behaviour of some of the radio-active elements. 
At first sight, the argument against such a state- 
ment would appear to be similar to one applicable 
to the “rare elements” of the earth series, 
lanthanum and its congeners, viz., that the methods 
of separation have not yet been found. But a little 
consideration must show this to be untenable. It 
is possible to apply electroscopic tests to. the radio- 
active elements capable of estimating their 
amount with an accuracy of, say, I -per cent. 
Suppose, then, that a certain process of separation 
is applied to a mixture of three elements, one of 
which is radio-active; and ‘suppose. that no 
diminution or increase is noticed in the relative 
amount of the radio-active element in either por- 
tion, it is legitimate to conclude that the radio- 
active element is inseparable from that element 
by the process used. By varying the process, if 
no separation is still effected, it appears a 
legitimate conclusion that separation by a chemical | 
process is impossible. This, of course, does not 
exclude separation by a physical process, sup- 
posing the atomic weights of the ‘“inseparable”’ 
elements to differ; for it is always possible to 
imagine the elements in a state of gas; and it is 
undeniable that a mixture of gases could be 
separated by diffusion into its constituents, pro- 
vided the gases possess different densities. 
NG. G22A VOL, ©3 | 
In this volume Mr. Soddy gives a diagram 
showing the position of the elements of high 
atomic weight; on the assumption that when a 
radio-active element loses an atom of helium, 
weighing 4, it joins a group of inseparable 
elements. Thus, to take an example :—Radium 
F, of atomic weight 210, in losing an atom of 
helium, forms a variety of lead of atomic weight 
206; similarly thorium C, of atomic weight 212, 
gives another element which passes as lead, in- 
separable from lead, of atomic weight 208; 
similarly radium C, of atomic. weight 214, in 
losing an atom of helium, yields radium D, of 
atomic weight 210, inseparable from lead. But 
this is not all; in the B ray changes, the element 
shifts its position by one place in the opposite 
direction to that caused by the loss of an 
a particle, without perceptibly changing its atomic 
weight; thus thorium D, losing a B corpuscle, or 
electron, shifts to another group—the lead group 
-—from the thallium group to which it actually 
belonged, without change of atomic weight. These 
processes are somewhat involved; but they appear 
to me to be a reasonable hypothesis, although 
further proof is desirable. This proof is evidently 
to be furnished by accurate determinations of the 
atomic weight of lead associated with thorium 
one hand and _ with radium 
minerals on the other. Supposing each variety 
of “lead” to be pure, the sample of ‘thorium 
lead” should have an atomic weight of about 208, 
while that of the “radium lead” should be about 
206. Determinations with this object in view are 
now in progress. The word “isotopic” is sug- 
gested as a fitting name for two elements, both 
occupying the same place in the periodic table. 
In his account of the theory, due credit is given 
to Fleck, Russell, and Fajans, the last of whom 
minerals on 
_ stated independently an almost identical hypo- 
' thesis. 
B 
