e effect in
a biotite crystal (Pl. XXV). Along what are apparently tubular
passages or cracks in the mica, a solution, rich in radioactive
substances, has moved; probably during the final consolidation of
the granite in which the mica occurs. A continuous and very
regular halo has developed along these conduits. A string of
halo-spheres may lie along such passages. We must infer that
solutions or gases able to establish the radioactive nuclei moved
along these conduits, and we are entitled to ask if all the
haloes in this biotite are not, in this sense, of secondary
origin. There is, I may add, much to support such a conclusion.

The widespread distribution of radioactive substances is most
readily appreciated by examination of sections of rocks cut thin
enough for microscopic investigation. It is, indeed, difficult to
find, in the older rocks of granitic type, mica which does not
show haloes, or traces of haloes. Often we find that every one of
the inclusions in the mica--that is, every one of the earlier
formed substances--contain radioactive elements, as indicated by
the presence of darkened borders. As will be seen presently the
quantities involved are generally vanishingly small. For example
it was found by direct determination that in one gram of the
halo-rich mica of Co. Carlow there was rather less than twelve
billionths of a gram of radium, We are

230

entitled to infer that other rare elements are similarly widely
distributed but remain undetectable because of their more stable
properties.

It must not be thought that the under-exposed halo is a recent
creation. By no means. All are old, appallingly old; and in the
same rock all are, probably, of the same, or neatly the same,
age. The under-exposure is simply due to a lesser quantity of the
radioactive elements in the nucleus. They are under-exposed, in
short, not because of lesser duration of exposure, but because of
insufficient action; as when in taking a photograph the stop is
not open enough for the time of the exposure.

The halo has, so far, told us that the additive law is obeyed in
solid media, and that the increased ionisation attending the
slowing down of the ray obtaining in gases, also obtains in
solids; for, otherwise, the halo would not commence its
development as a spherical shell or envelope. But here we learn
that there is probably a certain difference in the course of
events attending the immediate passage of the ray in the gas and