Final Products 579 
partially separated another radio-active element which he called 
actinium. It gives rise to an intermediate product actinium-X, 
which yields an emanation with the short half-life of 3°9 seconds, 
The emanation deposits two successive disintegration products ac- 
tinium-A and actinium-B. 
Evidence gradually accumulated that the amounts of actinium in 
radio-active minerals were, roughly at any rate, proportional to the 
amounts of uranium. This result pointed to a lineal connection 
between them, and led Boltwood to undertake a direct attack on the 
problem. Separating a quantity of actinium from a kilogramme of 
ore, Boltwood observed a growth of 8°5 x 10° gramme of radium in 
193 days, agreeing with that indicated by theory within the limits 
of experimental error’. We may therefore insert provisionally 
actinium and its series of derivatives between uranium and radium 
in the radio-active pedigree. 
Turning to the other end of the radium series we are led to ask 
what becomes of radium-F when in turn it disintegrates? What is 
the final non-active product of the series of changes we have traced 
from uranium through actinium and radium ? 
One such product has been indicated above. The a-ray particles 
appear to possess the mass of helium atoms, and the growth of helium 
has been detected by its spectrum in a tube of radium emanation. 
Moreover, helium is found occluded in most if not all radio-active 
minerals in amount which approaches, but never exceeds, the 
quantity suggested by theory. We may safely regard such helium 
as formed by the accumulation of a-ray particles given out by succes- 
sive radio-active changes. 
In considering the: nature of the residue left after the expulsion 
of the five a-particles, and the consequent passage of radium to 
radium-F we are faced by the fact that lead is a general constituent 
of uranium minerals. Five a-particles, each of atomic weight 4, 
taken from the atomic weight (about 225) of radium gives 205—a 
number agreeing fairly well with the 207 of lead. Since lead is more 
permanent than uranium, it must steadily accumulate, no radio-active 
equilibrium will be reached, and the amount of lead will depend on 
the age of the mineral as well as on the quantity of uranium present 
in it. In primary minerals from the same locality, Boltwood has 
shown that the contents of lead are proportional to the amounts of 
uranium, while, accepting this theory, the age of minerals with a given 
content of uranium may be calculated from the amount of lead they 
contain. The results vary from 400 to 2000 million years”. 
1 American Journal of Science, December, 1906. 
2 American Journal of Science, October, 1905, and February, 1907. 
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