ture--Radio-active
products in the earth's crust--Presence in air and soil
waters--Cosmical radio-activity 41

INDEX 53

A BRIEF ACCOUNT OF RADIO-ACTIVITY

CHAPTER I

DISCOVERY OF RADIO-ACTIVITY

The object of this brief treatise is to give a simple account of the
development of our knowledge of radio-activity and its bearing on
chemical and physical science. Mathematical processes will be omitted,
as it is sufficient to give the assured results from calculations
which are likely to be beyond the training of the reader. Experimental
evidence will be given in detail wherever it is fundamental and
necessary to a confident grasp of some of the marvelous deductions in
this new branch of science. Theories cannot be avoided, but the facts
remain while theories grow old and are discarded for others more in
accord with the facts.

The Beginning

As so often happens in the history of science, the opening up of this
new field with its fascinating disclosures was due to an investigation
undertaken for another purpose but painstakingly carried out with a
mind open to the truth wherever it might lead.

In 1895, Roentgen modestly announced his discovery of the _X_ rays.
This attracted immediate and intense interest. Among those who
undertook to follow up these phenomena was Becquerel, who, because of
the apparent connection with phosphorescence, tried the action of a
number of phosphorescent substances upon the photographic plate, the
most striking characteristic of the _X_ rays being their effect upon
such sensitive plates. In these experiments he obtained no results
until he tried salts of uranium, recalling previous observations of
his as to their phosphorescence. Distinct action was noted.
Furthermore, he proved that this had no connection with the phenomenon
of phosphorescence, as both uranic and uranous salts were active and
the latter show no phosphorescence. Becquerel announced his
discoveries in 1896 and this was the beginning of the new science of
radio-activity.

Radio-active bodies

The rays given off by uranium and its salts were found to differ from
the _X_ rays. They showed no appreciable variation in intensity, no
previous exposure of the substance to light was necessary, and neither
changes of temperature nor any other physical or chemical agency
affected them.

At first uranium and