Some other mineral may occasionally form the nucleus, but we never find any which is not known to be specially likely to contain a radioactive substance.  Another circumstance we notice.  The smaller this central nucleus the more perfect in form is the darkened circular area surrounding it.  When the circle is very perfect and the central mineral clearly defined at its centre we find by measurement that the radius of the darkened area is generally 0.033 mm.  It may sometimes be 0.040 mm.  These are always the measurements in biotite.  In other minerals the radii are a little different.

We see in the photograph (Pl.  XXIII, lower figure), much magnified, a halo contained in biotite.  We are looking at a region in a rock-section, the rock being ground down to such a thickness that light freely passes through it.  The biotite is in the centre of the field.  Quartz and felspar surround it.  The rock is a granite.  The biotite is not all one crystal.  Two crystals, mutually inclined, are cut across.  The halo extends across both crystals, but owing to the fact that polarised light is used in taking the photograph it appears darker in one crystal than in the other.  We see the zircon which composes the nucleus.  The fine striated appearance of the biotite is due to the cleavage of that mineral, which is cut across in the section.

The question arises whether the darkened area surrounding the zircon may not be due to the influence of the radioactive substances contained in the zircon.  The

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extraordinary uniformity of the radial measurements of perfectly formed haloes (to use the name by which they have long been known) suggests that they may be the result of alpha radiation.  For in that case, as we have seen, we can at once account for the definite radius as simply representing the range of the ray in biotite.  The furthest-reaching ray will define the radius of the halo.  In the case of the uranium family this will be radium C, and in the case of thorium it will be thorium C. Now here we possess a means of at once confirming or rejecting the view that the halo is a radioactive phenomenon and occasioned by alpha radiation; for we can calculate what the range of these rays will be in biotite, availing ourselves of Bragg’s additive law, already referred to.  When we make this calculation we find that radium C just penetrates 0.033 mm. and thorium C 0.040 mm.  The proof is complete that we are dealing with the effects of alpha rays.  Observe now that not only is the coincidence of measurement and calculation a proof of the view that alpha radiation has occasioned the halo, but it is a very complete verification of the important fact stated by Bragg, that the stopping power depends solely on the atomic weight of the atoms traversed by the ray.

We have seen that our examination of the rocks reveals only the two sorts of halo:  the radium halo and the thorium halo.  This is not without teaching.  For why not find an actinium halo?  Now Rutherford long ago suggested that this element and its derivatives were