Supplevient to ''Nature'' September i<„ 1923 



411 



than thirty in number, was soon disclosed and simply in- 

 terpreted on the transformation theory. The radioactive 

 elements provide us for the first time with a glimpse into 

 Nature's laboratory, and allow us to watch and study, 

 but not to control, the changes that have their origin in 

 the heart of the radioactive atoms. These atomic ex- 

 plosions involve energies which are gigantic compared 

 with those involved in any ordinary physical or chemical 

 process. In the majority of cases an a-particle is 

 expelled at high speed, but in others a swift electron 

 is ejected often accompanied by a 7-ray, which is a 

 \-ery penetrating X-ray of high frequency. The proof 

 that the a-particle is a charged helium atom for the 

 first time disclosed the importance of helium as one 

 of the units in the structure of the radioactive atoms, 

 and probably also in that of the atoms of most of the 

 ordinary elements. Not only then have the radio- 

 active elements had the greatest direct influence on 

 natural philosophy, but in subsidiary ways they have 

 provided us with experimental methods of almost equal 

 importance. The use of a-particles as projectiles with 

 which to explore the interior of the atom has definitely 

 exhibited its nuclear structure, has led to artificial 

 disintegration of certain light atoms, and promises to 

 yield more information yet as to the actual structure 

 of the nucleus itself. 



The influence of radioactivity has also extended to 

 yet another field of study of fascinating interest. We 

 have seen that the first rough estimates of the size 

 and mass of the atom gave little hope that we could 

 detect the effect of a single atom. The discovery that 

 the radioactive bodies expel actual charged atoms of 

 helium with enormous energy altered this aspect of 

 the problem. The energy associated with a single 

 a-particle is so great that it can readily be detected by 

 a variety of methods. Each a-particle, as Sir William 

 Crookes first showed, produces a flash of light easily 

 \ isible in a dark room when it falls on a screen coated 

 with crystals of zinc sulphide. This scintillation 

 method of counting individual particles has proved 

 invaluable in many researches, for it gives us a method 

 of unequalled delicacy for studying the effects of single 

 atoms. The a-particle can also be detected electrically 

 or photographically, but the most powerful and 

 beautiful of all methods is that perfected by Mr. 

 C. T. R. Wilson for observing the track through a gas, 

 not of an a-particle alone, but of any type of penetrating 

 radiation which produces ions or of electrified particles 

 along its path. The method is comparatively simple, 

 depending on the fact, first discovered by him, that 

 a gas saturated with moisture is suddenly cooled 

 vcich of the ions produced by the radiation becomes 

 the nucleus of a visible drop of water. The water- 

 drops along the track of the a-particle are clearly 

 \ isible to the eye, and can be recorded photographically, 

 'i'hese beautiful photographs of the effect produced by 

 single atoms or single electrons appeal, I think, greatly 

 to all scientific men. They not only afford con- 

 vincing evidence of the discrete nature of these particles, 

 but also give us new courage and confidence that the 

 scientific methods of experiment and deduction are to 

 be relied upon in this field of inquiry ; for many of the 

 essential points brought out so clearly and concretely 

 I these photographs were correctly deduced long before 

 uch confirmatory photographs were available. At the 



same time, a minute study of the detail disclosed in 

 these photographs gives us most valuable information 

 and new clues on many recondite effects produced by 

 the passage through matter of these flying projectiles 

 and penetrating radiations. 



In the meantime a number of new methods had been 

 devised to fix with some accuracy the mass of the 

 individual atom and the number in any given quantity 

 of matter. The concordant results obtained by widely 

 different physical principles gave great confidence in 

 the correctness of the atomic idea of matter. The 

 method found capable of most accuracy depends on 

 the definite proof of the atomic nature of electricity 

 and the exact valuation of this fundamental unit of 

 charge. We have seen that it was early surmised that 

 electricity was atomic in nature. This view was con- 

 firmed and extended by a study of the charges carried 

 by electrons, a-particles, and the ions produced in gases 

 by X-rays and the rays from radioactive matter. It 

 was first shown by Townsend that the positive or 

 negative charge carried by an ion in gases was invariably 

 equal to the charge carried by the hydrogen ion in 

 the electrolysis of water, which we have seen was 

 assumed, and assumed correctly, by Johnstone Stoney 

 to be the fundamental unit of charge. Various methods 

 were devised to measure the magnitude of this funda- 

 mental unit ; the best known and most accurate is 

 Millikan's, which depends on comparing the pull of 

 an electric field on a charged droplet of oil or mercury 

 with the weight of the drop. His experiments gave a 

 most convincing proof of the correctness of the electronic 

 theory, and gave a measure of this unit, the most 

 fundamental of all physical units, with an accuracy 

 of about one in a thousand. Knowing this value, we 

 can by the aid of electrochemical data easily deduce 

 the mass of the individual atoms and the number of 

 molecules in a cubic centimetre of any gas with an 

 accuracy of possibly one in a thousand, but certainly 

 better than one in a hundred. When we consider the 

 minuteness of the unit of electricity and of the mass 

 of the atom, this experimental achievement is one of 

 the most notable even in an era of great advances. 



The idea of the atomic nature of electricity is very 

 closely connected with the attack on the problem of 

 the structure of the atom. If the atom is an electrical 

 structure it can only contain an integral number of 

 charged units, and, since it is ordinarily neutral, the 

 number of units of positive charge must equal the 

 number of negative. One of the main difficulties in 

 this problem has been the uncertainty as to the relative 

 part played by positive and negative electricity in the 

 structure of the atom. We know that the electron 

 has a negative charge of one fundamental unit, while 

 the charged hydrogen atom, whether in electrolysis 

 or in the electric discharge, has a charge of one positive 

 unit. But the mass of the electron is only 1/1840 of 

 the mass of the hydrogen atom, and though an ex- 

 tensive search has been made, not the slightest evidence 

 has been found of the existence of a positive electron 

 of small mass like the negative. In no case has a 

 positive charge been found associated with a mass less 

 than that of the charged atom of hydrogen. This 

 difference between positive and negative electricity is 

 at first sight very surprising, but the deeper we pursue 

 our inquiries the more this fundamental difference 



