tube.  The dark central part is the capillary.  The tubular halo surrounds it.  This experiment has, however, been anticipated by some scores of millions of years, for here is the same 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

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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 in the solid.  In the former, initial recombination may obscure the intense ionisation near the end of the range.  We can only detect the true end-effects by artificially separating the ions by a strong electric force.  If this recombination happened in the mineral we should not have the concentric spheres so well defined as we see them to be.  What, then, hinders the initial recombination in the solid?  The answer probably is that the newly formed