oncluding this outline of the main facts of radio-activity, it is
of interest to discuss briefly the presence of radio-active material
on this planet and in the stars. Facts enough have been gathered to
show the probable universality of this phenomenon of radio-activity.
Whether this means solely the disintegration of the uranium and
thorium atoms, or whether other elements are also transformed under
the intensity of the agencies at work in the universe, is of course a
question as yet unsolved.

Radio-active Products in the Earth's Crust

The presence of uranium and thorium widely distributed throughout
the crust of the earth would lead to the conclusion that their
disintegration products would be found there also. Various rocks of
igneous origin have been examined revealing from 4.78 x 10^{-12}
to 0.31 x 10^{-12} grams of radium per gram of the rock. Aqueous
rocks have shown a lesser amount, ranging from 2.92 x 10^{-12} to
0.86 x 10^{-12} grams. As the soil is formed by the decomposition
of these rocks, radium is present in varying amounts in all kinds of
soil.

Presence in Air and Soil Waters

As radium is transformed into the gaseous emanation, this will escape
wherever the soil is not enclosed. For instance, a larger amount of
radio-activity is found in the soil of caves and cellars than in open
soils. If an iron pipe is sunk into a soil and the air of the soil
sucked up into a large electroscope, the latter instrument will show
the effect of the rays emitted and will measure the degree of
activity. Also the interior of the pipe will receive a deposit of the
radio-active material and will show appreciable radio-activity after
being removed from the soil.

This radium emanation is dissolved in the soil waters, wells, springs,
and rivers, rendering them more or less radio-active, and sometimes
the muddy deposit at the bottom of a spring shows decided
radio-activity.

The emanation also escapes into the air so that many observations made
in various places show that the radium emanation is everywhere present
in the atmosphere. Neither summer nor winter seems to affect this
emanation, and it extends certainly to a height of two or three miles.
Rain, falling through the air, dissolves some of the emanation, so
that it may be found in freshly-fallen rain water and also in
freshly-fallen snow. Radio-active deposits are found upon electrically
charged wires exposed near the earth's surface.

As helium is the re