Part 1 (1/2)

A Brief Account of Radio-activity.

by Francis Preston Venable.

PREFACE

I have gathered the material for this little book because I have found it a necessary filling out of the course for my cla.s.s in general chemistry. Such a course dealing with the composition and structure of matter is left unfinished and in the air, as it were, unless the marvellous facts and deductions from the study of radio-activity are presented and discussed. The usual page or two given in the present text-books are too condensed in their treatment to afford any intelligent grasp of the subject, so I have put in book form the lectures which I have hitherto felt forced to give.

Perhaps the book may prove useful also to busy men in other branches of science who wish to know something of radio-activity and have scant leisure in which to read the larger treatises.

It is needless to say that there is nothing original in the book unless it be in part the grouping of facts and order of their treatment. I have made free use of the writings of Rutherford, Soddy, and J. J. Thomson, and would here express my debt to them--just a part of that indebtedness which we all feel to these masters. I wish also to acknowledge my obligations to Professor Bertram B. Boltwood for his helpful suggestions in connection with this work.

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 a.s.sured 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, Rontgen 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 phosph.o.r.escence, tried the action of a number of phosph.o.r.escent 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 phosph.o.r.escence. Distinct action was noted.

Furthermore, he proved that this had no connection with the phenomenon of phosph.o.r.escence, as both uranic and uranous salts were active and the latter show no phosph.o.r.escence. 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 its compounds were the only known source of these new radiations, but many other substances were examined and two years later thorium and its compounds were added to the list. In general the discharging action seemed about the same. Other elements and ordinary substances show a minute activity. Only pota.s.sium and rubidium have a greater activity than this, and theirs is only about one-thousandth that of uranium.

An Atomic Property

In the examination of uranium and thorium compounds it was found that the activity was determined by the uranium and thorium present; it was proportto the amount ofional these elements present and independent of the nature of the other elements composing the compound. The conclusion was, therefore, that the activity was an inherent property of the atoms of uranium and thorium, that is, an atomic property. This was a long step forward and introduced into science the conception of a new property of matter, or at least of certain forms of matter.

Discovery of New Radio-active Bodies

In examining a large number of minerals containing uranium and thorium, Mme. Curie made the important observation that many of these were more active than the elements themselves. In measuring the activity she made use of the electrical method which will be described later. In the following table giving her results for uranium minerals the numbers under _i_ give the maximum current in amperes. They serve simply for comparison.