PROF. J. .TORY ON THE GENESIS OF PLEOCHROTC HALOES. 
77 
measured with a considerable degree of accuracy and I do not think there is any 
doubt of the misfit. The emanation halo, which is also measurable with reliable 
accuracy, shows a small misfit in the opposite sense ; the halo-radius in the mica, 
being too small for the halo-radius in air. But in this case I have assumed that the 
a-rays leave the very surface of the nucleus, and I have accordingly deducted the 
entire radius of the latter. This assumption may not be justified. If the emanation 
became deeply absorbed in the nucleus a lesser deduction would be correct. The 
mean nuclear radius is 0’0009 mm. Restoring one-half this to- the radial measure¬ 
ments of the halo we find that its axial radius falls at 3‘36 cm. assuming the 
conversion factor 2075. This gives g-ood agreement. In short we are not in a 
position to lay stress upon the apparent misfit in the case of this halo. 
We are entitled to ask for the possible explanation of the misfit of the inner 
features of the uranium halo. The easiest answer would obviously be that some 
addition to our knowledge of the ionisation curve was required which would have 
the effect of modifying the inner features of the curve. The ionisation curve of IJ L 
and Uo has been investigated by Geiger and Nijttall'" and found to agree in range 
with what would be expected from the logarithmic law connecting the range with 
the transformation constant. There would, therefore, appear to lie no room for error 
here. If, then, the discrepancy is to be sought in the plot of the halo in air it would 
seem as if we must look for some element at present omitted from the series. But 
we are in the difficulty that the introduction of such an element must disturb the 
transformation constants of the recognised elements and these transformation 
constants have been found to be in good agreement with the logarithmic law already 
referred to. 
Tire only other suggestion I can offer is that of an actual change in the range ot 
the a-rays, since the remote period when the haloes were formed. The age of the 
Co. Carlow (Ballyellen) rock is late Silurian or early Devonian; that of the Yagnay 
rock, Carboniferous (pre-Stephanian), or possibly very much older. The Cornish 
granite is Carboniferous in age. We would have to assume that the ranges of the 
uranium rays were formerly longer than they now are, or that a proportion of 
uranium atoms then existed having a longer range. As we know that isotopic 
elements may possess very various radioactive properties, there does not seem to be 
any objection to this hypothesis from the chemical point of Anew, the atoms of 
longer and shorter range quite possibly obeying the same chemical influences in the 
processes attending magmatic differentiation. 
This view would involve, of course, the transformation constants ; the atoms of 
longer a-range possessing the shorter longevity. We are not in a position to state 
that the observed discrepancy between the ranges inferred from the halo in mica and 
those recognised to-day in air is confined to the early uranium atoms. The 
observations suggest that the radioactive properties of atoms derived from the latter 
* ‘Phil. Mag.,’ March, 1912. 
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