PROF. J. JOLY ON THE GENESIS OF PEEOCHROIC HALOES. 
71 
about 2'2 and 3 ’5 cm. Their simultaneous development is explained by the nearly 
equal ionisation responsible for each ; the somewhat greater ionisation of the outer 
maximum counteracting a certain amount of inward concentration recognisable in 
halo-development. The inner region of the first ring darkens up under the influence 
of the fairly strong ionisation shown within the first maximum. The somewhat 
uncertain third ring is probably referable to the prominence on the curve at 4'5 cm. 
The pupil may extend to a radial distance corresponding to 5'8 cm. Beyond this the 
penumbra due to ThC 3 appears. 
We find, therefore, that the thorium halo develops in a manner quite in keeping 
with the form of the curve of ionisation in air representing the added activities of the 
known rays emitted by members of the thorium family. How far the location of the 
several observed features agrees with the ionisation curve will presently appear. I 
now quote some measurements made on thorium haloes in various stages of 
development. The haloes are all from the mica of Yagnay, Vosges. The dimensions 
under r x refer to the first ring. A few readings to the axis of the ring are given. 
Most of the readings are taken to the outside boundary, this being the most 
definite feature of the ring. The readings applying to the second ring are in the 
column headed r 2 and are taken to the central darkening ; or, failing that, to the 
central line or axis of the ring as far as this can be estimated by eye. One reading 
to the centre of the third ring, r 3 , is given. The radial dimension of the pupil in 
well darkened haloes is next given. These haloes vary much in depth of colour. 
The extreme boundary of the halo appears in the last column. 
Compound Uranium-Thorium Haloes. 
It is well known that thorium-bearing minerals in most if not in all cases contain 
uranium. It might be expected, therefore, that the occurrence of compound haloes 
due to the joint action of the a-rays of both these families of radioactive elements 
would be frequent. In order the better to detect the presence of such haloes I 
plotted the ionisation curve due to an equal admixture of the parent substances. The 
distinctive feature of this curve was found to be the existence of a double outer ring : 
the outer ring due to ThCh containing within it the ring due to RaC. This was an 
easily recognisable feature. As regards internal features the second maximum, which 
is so conspicuous a feature of the thorium ionisation curve, is nearly obliterated. The 
compound halo would start with a strong ring at the distance of 2'3 cm. in air. 
In the Leinster granite thorium minerals appear to be absent. But in the Vosges 
granite there are found side by side both thorium and uranium haloes. A careful 
watch for the compound halo was kept when examining this rock. However, only one 
quite definite example of a compound halo was observed. It was recognised by the 
double penumbra. The measured radii of the two rings were found to be quite 
correct; and as the strength of development was rather greater for the thorium ring 
it may be inferred that this substance predominated. Unfortunately the reference 
