A Brief Account of Radio-activity Part 1

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.

_i_ Pitchblende from Joachimsthal 7.0 10^{-11} Clevite 1.4 10^{-11} Chalcolite 5.2 10^{-11} Autunite 2.7 10^{-11} Carnot.i.te 6.2 10^{-11} Uranium 2.3 10^{-11} Uranium and pota.s.sium sulphate 0.7 10^{-11} Uranium and copper phosphate 0.9 10^{-11}

The last three are pure uranium and compounds of that element given for comparison with the first five, which are naturally occurring minerals. The last compound has the same composition as chalcolite and is simply the artificially prepared mineral. It has the activity which would be calculated from the proportion of uranium present, the copper and phosphoric acid contributing no activity.

Since the activity is not dependent upon the composition but upon the amount of uranium present, the activity in all of the minerals should be less than that of uranium. On the contrary, it is several times greater. Natural and artificial chalcolite also show a marked difference in favor of the former. The supposition was a natural one, therefore, that these minerals contained small quant.i.ties of an element, or elements, undetected by ordinary a.n.a.lysis and having a much greater activity than uranium. Similar results were obtained in the examination of thorium minerals and thorium salts.

Discovery of Polonium

Following up this supposition, M. and Mme. Curie set themselves the task of separating this unknown substance. Starting with pitchblende, a systematic chemical examination was made. This is an exceedingly complex mineral, containing many elements. The processes were laborious and demanded much time and minute care. They need not be described here. It is sufficient to say that along with bis.m.u.th a very active substance was separated, to which Mme. Curie gave the name of polonium for Poland, her native land. Its complete isolation is very difficult and sufficient quant.i.ties of the pure substance have not been obtained to determine its atomic weight and other properties, but some of the lines of its spectrum have been determined. Chemically it is very closely a.n.a.logous to bis.m.u.th.

Discovery of Radium

In a similar manner a barium precipitate was obtained from pitchblende which contained a highly active substance. The pure chloride of this body and barium can be prepared together and then separated by fractional crystallization. To the new body thus found the name of radium was given. It is similar in chemical properties to barium. Its atomic weight has been determined by several careful investigators and is accepted as 226. Its spectrum has been mapped and its general properties are known. It is a silvery white, oxidizable metal. In one ton of pitchblende about 0.2 gram of radium is present; this is about 5000 times greater than the amount of polonium present. The activity of the products was depended upon as the guide in these separations.

The radium found is relatively enormously more active than the pitchblende or uranium.

Other Radio-active Bodies Found

In the above separations use was made of relations.h.i.+ps to bis.m.u.th and barium. Similarly, by taking advantage of chemical relations.h.i.+p to the iron group of elements, another body was partially separated by Debierne, to which he gave the name actinium. Boltwood discovered in uranium minerals the presence of a body which he named ionium, and which is so similar to thorium that it cannot be separated from it.

It, however, far exceeds thorium in activity.

The lead which is present in uranium and thorium minerals--apparently in fairly definite ratio to the amount of uranium and thorium--is found, on separation and purification, to possess radio-active properties. This activity is due to the presence of a very small proportion of an active const.i.tuent called radio-lead, which has chemical properties identical with those of ordinary lead. The bulk of the lead obtained from radio-active minerals differs in atomic weight from ordinary lead and appears also to be different according to whether its source is a thorium or a uranium mineral.

A large number of other radio-active substances have been separated and some of their properties determined, but these were found by different means and will be noted in their proper place. They number in all more than thirty. The sources or parents of these are the original uranium or thorium, and the products form regular series with distinctive properties for each member.

CHAPTER II

PROPERTIES OF THE RADIATIONS

The activity of these radio-active bodies consists in the emission of certain radiations which may be separated into rays and studied through the phenomena which they cause.

Ionization of Gases

One of these phenomena is the power of forming ions or carriers of electricity by the pa.s.sage of the rays through a gas, thus ionizing the gas. The details of an experiment will serve to make the meaning of this ionization clear.

[Ill.u.s.tration: FIG. 1.--IONIZATION OF GASES.]

When this apparatus is set up a minute current will be observed without the introduction of any radio-active matter. This, as Rutherford says, has been found due mainly to a slight natural radio-activity of the matter composing the plates. If radio-active matter is spread on plate _A_, which is connected with one pole of a grounded battery, and if plate _B_ is connected with an electrometer which is also connected with the earth, a current is caused which increases rapidly with the difference of potential between the plates, then more slowly until a value is reached that changes only slightly with a larger increase in the voltage.

According to the theory of ionization, the radiation produces ions at a constant rate. The ions carrying a positive charge are attracted to plate _B_, while those negatively charged are attracted to plate _A_, thus causing a current. These ions will recombine and neutralize their charges if the opportunity is given. The number, therefore, increases to a point at which the ions produced balance the number recombining.