sulting product of the alpha particles emitted by
the emanation and other radio-active bodies, it is found in the soil
air, soil waters, and atmosphere.

Average measurements of the radio-activity of the atmosphere have led
to the calculation that about one gram of radium per square kilometer
of the earth's surface is requisite to keep up the supply of the
emanation.

A number of estimates have been given as to the heat produced by the
radio-active transformations going on in the material of this planet.
Actual data are scarce and mere assumptions unsatisfactory, so little
that is worth while can be deduced. It is possible that this source of
heat may have an appreciable effect upon or serve to balance the
earth's rate of cooling.

Cosmical Radio-activity

Meteorites of iron coming from other celestial bodies have not shown
the presence of radium. Aerolites or stone meteorites have been found
to contain as much as similar terrestrial rock. Since the sun
contains helium and some stars show its presence as predominating,
this suggests the presence of radio-active matter in these bodies. In
addition, the spectral lines of uranium, radium, and the radium
emanation have been reported as being found in the sun's spectrum and
also in the new star, _Nova Geminorum 2_. These observations await
further investigation and confirmation. So far as the sun's
chromosphere is concerned, the possible amount of radium present would
seem to be very small. If this is true, radio-active processes could
have little to do with the sun's heat. The statement is made by
Rutherford that indirect evidence obtained from the study of the
aurora suggests that the sun emits rays similar in type to the alpha
and beta rays. Such rays would be absorbed, and the gamma rays
likewise, in passing through the earth's atmosphere and so escape
ordinary observation. All of this is but further evidence of the unity
of matter and of forces in the universe.

INDEX

Actinium, discovery of, 6

Activity, induced, 17

Alpha particles, effect of loss on Atomic Weight, 45
electrical charge of, 26
form helium, 27
nature of, 25
penetrating power of, 39
position of element changed by its loss, 46
recoil, 39
scattering of, 38
solid, 26

Atom, constitution of, 36
Kelvin's, 37
models of, 37
Rutherford's, 37

Atomic number, determination of, 43

Becquerel's experiments, 2

Beta particl