APRIL 25, 1912] 
got by the following | 
at its present price | 
For the element, | 
750,0001. per ounce. | 
To represent 
indicated by these curves can be 
consideration. Radium bromide 
costs about 16]. per milligram. 
radium, this is at the rate of 
Fig. 3 
represents a diagram 2 ft. high. 
48 
S 8 3 
QUANTITY OF RADIUM 
FIG. 3. 
a pennyworth of radium on this scale would require | 
a diagram more than 6000 ft. high, whereas to repre- 
sent Keetman’s curve (I"ig. 2) would require one as 
high as St. Paul’s Cathedral. 
These results, therefore, confirm absolutely the view 
that uranium does not produce 
radium directly. As Rutherford 
first showed, if ionium is the only 
long-lived radio-element between 
uranium and radium, the growth 
of radium from uranium must 
initially be proportional to the 
square of the time, and should 
be represented by the equation 
R=6x10-8AT?, where R is the 
radium formed per kilogram of 
uranium, T is the time in years, 
and 1/A is the period of ionium. 
Hence, if uranium is the primary 
parent of radium, it is to be ex- 
pected that the rate of growth of 
radium from the preparations will 
increase as time goes on accord- 
ing to some power of the time 
higher than unity. As Fig. 3 
shows, there is still no evidence 
of this increase of slope in any of 
the preparations. This indicates, 
either that the period of ionium 
must be enormously long, or that 
several intermediate long-lived 
members intervene. If ionium is 
the only intervening member a 
minimum possible limit to its 
period may be arrived at by 
applying the above equation to 
the results. If it is assumed that the growth 
observed is due to uranium and that no ionium was 
initially present, the minimum periods calculated in 
the several experiments are follow :—No. I., | 
28,000 years; No. II., 41,400 years; No. III., 80,000 
NO. 2217, VOL. 89] 
x 1078 EQUILIBRIUM QTY 
as 
NATURE 
205 
years; and No. IV., 69,200 years. Since, in all, cer- 
tainly some of the growth is due to ionium initially 
present, the period of ionium must certainly be greater 
than the longest of these periods. We may safely 
conclude, if ionium is the only intermediate member, 
that its period is at least 100,000 
years. This is forty times longer 
than the period of radium itself. 
Entirely independent confirmation 
of this conclusion was obtained in 
another The 
way. gap in our 
knowledge is, strictly speaking, not 
between uranium and ionium, for 
the direct product of uranium is well 
known, and is called uranium X. 
It gives B-rays alone in disinte- 
grating and has a period of only 
35°5 days, so that in all the pre- 
ceding work it has not been neces- 
sary to take it into account. It 
would retard the growth of radium 
inappreciably. But, if the view is 
right, the product of uranium X 
must be ionium, which gives a-rays. 
Concomitantly with the rapid decay 
of the intense S-rays of uranium X 
there should occur a growth of 
a-rays due to the ionium produced. 
Whether these a-rays can be actually 
detected will depend on the period 
of ionium. From experiments on 
the uranium X separated from 50 
kilograms of uranium nitrate no 
growth of a-radiation, concomitant with or subse- 
quent to the decay of the f8-radiation, could be de- 
tected, and from these negative results the minimum 
period ionium can possess, if it is the only long-lived 
intermediate product, is 30,000 years. 
The question arose whether by any means an upper 
limit, or maximum value, for the period of ionium 
could be assigned. By the law already discussed there 
must be many times as much ionium as radium in 
uranium minerals, and if the actual ratio were known 
