PLEOCHROIC HALOES—JOLY. 317 
product of the number of each sign present in unit volume. Here 
the numbers are very great and the volume very small. The ionic 
density is therefore high, and recombination very rapidly removes 
the ions after they are formed. We see here a peculiarity of the 
ionization effected by alpha rays. It is linear in distribution and 
very local. Much of the ionization in gases is again undone by 
recombination before diffusion leads to the separation of the ions. 
This ‘“‘initial recombination” is greatest toward the end of the path 
of the ray where the ionization is a maximum. Here it may be so 
effective that the form of the curve is completely lost unless a very 
large electromotive force is used to separate the ions when the 
ionization is being investigated. 
We have now reviewed recent work at sufficient length to under- 
stand 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 ionize 
the atoms which it meets in its path. It spends its energy in the 
act of ionizing them, coming to rest, when it moves in air, in a few 
centimeters. Its particular initial velocity depends upon which of 
the radioactive elements 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 
ionizing 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 ionization 
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 fede our conception of 
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 pro- 
pose 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 fhend) quite irregularly. The most usual appearance is that of 
a circular area darker in color than the surrounding mineral. The 
radii of these little disk-shaped marks when well defined are found 
