PLEOCHROIC HALOES—JOLY. 325 
then 150 millions of rays built up the halo. If geological time since 
the Devonian is 400 millions of years, then 1,200 millions of alpha 
rays are concerned in its genesis. The number of ions involved, of 
course, greatly exceeds these numbers. A single alpha ray fired 
from radium C will produce 2.37 x 10° ions in air. 
But haloes may be found quite clearly defined and fairly dark out 
to the range of the emanation ray and derived from much lesser 
quantities of radioactive materials. Thus a zircon nucleus with a 
diameter of but 3.4X10-* centimeter formed a halo strongly 
darkened within, and showing radium A and radium C as clear 
smoky rings. Such a nucleus, on the assumption made above as 
to its radium content, expels one ray ina year. But, again, haloes 
are observed with less blackened pupils and with faint ring due to 
radium C, formed round nuclei of rather less than 2 x 10-* centimeter 
diameter. Such nuclei would expel one ray im five years. And 
even lesser nuclei will generate in these old rocks haloes with their 
earlier characteristic features clearly developed.- In the case of 
the most minute nuclei, if my assumption as to the uranium content 
is correct, an alpha ray is expelled, probably, no oftener than once 
_ in a century, and possibly at still longer intervals. 
The equilibrium amount of radium contained in some nuclei may 
amount to only a few atoms. Even in the case of the larger nuclei 
and more perfectly developed haloes the quantity of radium involved 
is many millions of times less than the least amount we can recognize 
by any other means. But the delicacy of the observation is not 
adequately set forth in this statement. We can not only tell the 
nature of the radioactive family with which we are dealing, but 
we can recognize the presence of some of its constituent members. 
I may say that it is not probable the zircons are richer in radium 
than I have assumed. My assumption involves about 3 per cent 
of uranium. I know of no analyses ascribing so great an amount 
of uranium to zircon. The variety cyrtolite has been found to 
contain half this amount, about. But even if we doubled our 
estimate of radium content, the remarkable nature of our conclusions 
is hardly lessened. 
It may appear strange that the ever-interesting question of the 
earth’s age should find elucidation from the study of haloes. Never- 
theless the subjects are closely connected. The circumstances are 
as follows: Geologists have estimated the age of the earth since 
denudation began, by measurements of the integral effects of de- 
nudation. These methods agree in showing an age of about 10° 
years. On the other hand, measurements have been made of the 
accumulation in minerals of radioactive débris—the helium and lead— 
and results obtained which, although they do not agree very well 
among themselves, are concordant in assigning a very much greater 
