68 The N.Z. Journal of Science and Technology. [Mar. 
of Professor Page (“A Century’s Progress in Physics,” Amer. Jour. Science , 
July, 1918) : “ That science is stagnant whose only function is to collect, 
classify, and correlate vast stores of experimental data. The sign of 
vitality is the existence of clearly defined and fundamental problems, any 
possible solution of which seems irreconcilable with the most basic truths 
of the science in question. The greater the paradox grows, the more 
certain is the advent of a new point of view which will bring one step 
nearer the comprehensive picture of nature which is the goal of natural 
philosophy.” 
Perhaps we may find in the present apparent impasse a way of progress 
in the suggestion put forward by that most ingenious of geological physicists, 
Professor Joly, which if confirmed by further investigation may go far to 
reconcile the physical and astronomical age-estimates with the period of 
100 millions of years that Professor Joly with other geologists has shown 
on chemico-geological considerations to be fairly probable ( Nature , Aug. 2 
and 9, 1917, esp. pp. 477-79). The element thorium goes through a series 
of changes analogous to those of uranium, yielding also as a final product 
a form of lead. Yet if we measure the age of minerals by the ratio of 
thorium to lead we find (as was shown by Professor Soddy in the case of 
thorite from Ceylon) that the age of pre-Cambrian rocks appears to be only 
140 millions of years, or a tenth of that indicated by the ratio of uranium 
to lead. Why, then, this further contradiction ? The weak point appears 
to be the fundamental assumption of the invariability of the rate of 
disintegration of the uranium atom ; and this it is that Professor Joly 
challenges. His evidence is extremely interesting. Every little crystal of 
uranium-bearing mineral (usually zircon) that occurs as an inclusion in the 
brown-mica or biotite of ancient granitic rocks is found to be surrounded by 
a dark “ halo ” (containing one or more distinctly darker rings) of altered 
biotite, produced by the ionization of the mica-molecule as a result of the 
action of the radiations from decaying uranium in the zircon. The diameter 
of this halo, and in particular of one of its rings, is determined by the range 
of the alpha particles emitted ; but, though the dimensions in question are 
very small, there seems to be no doubt possible but that the diameter of 
the ring produced by the alpha particles is greater than it would have been 
had it been produced by alpha particles emitted at the same speed as they 
are emitted from the mineral at the present time. Now, the range of the 
alpha particles emitted from uranium is greater the more rapid is the atomic 
transformation by which it is produced, and “ there seems no apparent 
escape from the conclusion that the ring . . . has been formed by rays 
of greater range than the average range of the rays now emitted.” “ The 
measurements of the uranium halo admit of the interpretation that they 
indicate the failure of uranium-derived lead as a true indicator of geological 
time. For if the range of 'Ui was indeed in remote times longer than it now 
is, then we must suppose that its rate of decay was at that period faster 
than it is to-day.” The haloes produced by radiation around minerals 
containing thorium do not appear to show such marked discrepancies 
between the range, calculated from the present molecular activities, and 
the actual measurements of the diameters of halo-rings. The age-estimate 
derived from the ratio of thorium to lead should therefore give a closer 
approximation to the true figure. “ I am far from contending,” said 
Professor Joly, “ that this view is free from difficulties. On the other 
hand, our ignorance of the mode of origin of radio-activity and of its 
possibilities is very considerable.” 
