We have now reviewed recent work at sufficient length to understand something of the nature of the most important advance ever made in our knowledge of the atom.  Let us glance briefly at what we have learned.  The radioactive atom in sinking to a lower atomic weight casts out with enormous velocity an atom of helium.  It thus loses a definite portion of its mass and of its energy.  Helium which is chemically one of the most inert of the elements, is, when possessed of such great kinetic energy, able to penetrate and ionise the atoms which it meets in its path.  It spends its energy in the act of ionising them, coming to rest, when it moves in air, in a few centimetres.  Its initial velocity depends upon the particular radioactive element which has given rise to it.  The length of its path is therefore different according to the radioactive element from which it proceeds.  The retardation which it experiences in its path depends entirely upon the atomic weight of the atoms which it traverses.  As it advances in its path its effectiveness in ionising the atom rapidly increases and attains a very marked maximum.  In a gas the ions produced being much crowded together recombine rapidly; so rapidly that the actual ionisation may be quite concealed unless a sufficiently strong electric force is applied to separate them.  Such is a brief summary of the climax of radioactive discovery:—­the birth, life and death of the alpha ray.  Its advent into Science has altered fundamentally our conception of

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matter.  It is fraught with momentous bearings upon Geological Science.  How the work of the alpha ray is sometimes recorded visibly in the rocks and what we may learn from that record, I propose now to bring before you.

In certain minerals, notably the brown variety of mica known as biotite, the microscope reveals minute circular marks occurring here and there, quite irregularly.  The most usual appearance is that of a circular area darker in colour than the surrounding mineral.  The radii of these little disc-shaped marks when well defined are found to be remarkably uniform, in some cases four hundredths of a millimetre and in others three hundredths, about.  These are the measurements in biotite.  In other minerals the measurements are not quite the same as in biotite.  Such minute objects are quite invisible to the naked eye.  In some rocks they are very abundant, indeed they may be crowded together in such numbers as to darken the colour of the mineral containing them.  They have long been a mystery to petrologists.

Close examination shows that there is always a small speck of a foreign body at the centre of the circle, and it is often possible to identify the nature of this central substance, small though it be.  Most generally it is found to be the mineral zircon.  Now this mineral was shown by Strutt to contain radium in quantities much exceeding those found in ordinary rock substances.

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