Novembers, 1919] 



NATURE 



225 



city, and the success which has been obtained is 

 due in the main to the fact that the methods by 

 which we can detect the existence and follow the 

 behaviour of these charged particles are almost 

 infinitely more powerful than those which are 

 available when the particles are uncharged. We 

 can by the aid of their charges detect the presence 

 of a few thousand atoms, while the most delicate 

 methods of chemical analysis will scarcely detect a 

 million million. Again, when an atom or molecule 

 is charged we can by acting upon it by electrical 

 forces increase its energy a million-fold, and thus 

 enable it to produce effects by which its presence 

 can be detected. We obtain in this way very 

 powerful and accurate methods for measuring 

 some of the fundamental constants associated 

 with atoms and molecules. We know now, for 

 example, with great precision the masses 

 of the molecules of the different gases ; we 

 owe this to the study of their electrical 

 properties. 



Again, the study of the positive rays has shown 

 that all the atoms of an element have to a very 

 high degree of approximation the same mass, and 

 has disposed of the idea that the atomic weight 

 only represented an average value taken over a 

 considerable range. The positive rays, too, have 

 demonstrated the existence in most gases of both 

 atoms and molecules ; not only have they shown 

 that atoms exist, they have also proved the inde- 

 pendent existence of the radicles of organic 

 chemistry such as CH, CH^, CH3. These rays 

 will, I think, in the future play a considerable part 

 in the determination of the atomic weight of those 

 elements which can exist in the gaseous form, as 

 they furnish a method which is independent of im- 

 purities, and can distinguish between "isotopes," 

 should such exist. The rays provide a powerful 

 method for detecting new elements and compounds, 

 as they demand only an infinitesimal amount of 

 material and the atomic weight of the new body 

 can be calculated at once from the position of its 

 line in the positive ray spectrum. As a side issue 

 the rays show the complexity of the conditions 

 when electricity passes through compound gases. 

 I have found cases in which there were as many 

 as thirty-seven different types of positive carriers 

 at work simultaneously. 



The convection of negative electricity presents a 

 remarkable contrast, for one of the most striking 

 results of the study of the electrical properties of 

 gases is that at very low pressures the carriers of 

 negative electricity are not atoms or molecules, but 

 electrons, the mass of which is only about 

 I / 1700 of that of the smallest known atom, that 

 of hydrogen ; these carriers are unaltered in char- 

 acter whatever changes may take place in the 

 nature of the gas through which the electricity is 

 passing. These electrons can be obtained from 

 atoms of every kind, so that they form an integral 

 part of the normal atom. The number of elec- 

 trons in an atom which are not fixed too rigidly to 

 be shaken when struck by Rontgen rays has been 

 determined, and it has been found that the number 

 NO. 2610, VOL. 104] 



of such electrons in an atom of any element is 

 equal to the atomic number of the element. The 

 positive rays show that the atoms of elements 

 other than hydrogen which occur in these rays 

 must contain more than one electron, for atoms 

 which have lost two or more electrons are a com- 

 mon feature in these rays ; mercury atoms have 

 been observed which have lost as many as eight 

 electrons. The speed which the electrons may 

 attain is very great ; some of the electrons emitted 

 by radio-active substances (the /3-rays) travel at a 

 speed only a few per cent, less than that of 

 light. 



The source of the mass of the electrons is in- 

 teresting ; it was known before they were dis- 

 covered that a charged body had in virtue of its 

 charge a larger mass than an uncharged one, the 

 difference increasing as the size of the body 

 diminished. The result at that time looked very 

 academic, as even molecules were far too large 

 for the effect to be appreciable ; the result became 

 of practical importance when electrons (the linear 

 dimensions of which are only about one-hundred- 

 thousandth part of those of atoms) were dis- 

 covered ; and the experiments indicate that the 

 whole of the mass of an electron is due to its 

 charge. Mass of this kind depends upon the 

 velocity and becomes infinite when the velocity 

 is that of light. The mass of the electrons accounts, 

 however, for only a minute fraction of that of the 

 atom of which they form a part. 



Since we know the number of electrons in an 

 atom, the problem of finding the structure of the 

 atom is that of finding the configuration of these 

 electrons when they are in equilibrium under their 

 mutual repulsions and whatever forces may be 

 exerted upon them by the positive charges. The 

 solution of this problem would give representa- 

 tions of the structure of the atoms of the various 

 elements. The consideration of the positions of 

 equilibrium when two such atoms of the same or 

 different kinds are brought near together would 

 lead to clear views as to what constitutes chemical 

 combination and the conditions under which it is 

 possible. This is one of the problems which cgll 

 most urgently for solution. It must be noticed, 

 however, that we cannot explain the properties of 

 the atoms of the elements by a system of positive 

 and negative point charges exciting forces varying 

 inversely as the square of the distance. These 

 would not give rise to systems of atoms sharply 

 limited to definite and distinct types, but to systems 

 passing continuously from one type to another. 

 To get the requisite definiteness in the model atom 

 we must introduce some other condition, such, for 

 example, as that the force between the positive 

 and negative forces is not always an attraction 

 varying inversely as the square of the distance, but 

 that it changes from attraction to repulsion at defi- 

 nite distances (such distances giving a length to 

 measure the size of the atom), or we may assume 

 some condition such as is imposed by the quantum 

 theory, which rules out all but a small fraction of 

 the solutions otherwise possible. 



