November 6, 1919] 



NATURE 



227 



proof of the atomic nature of electricity and the 

 accurate measure of the fundamental atomic and 

 molecular magnitudes are two of the greatest 

 triumphs of the new era. 



One of the most important properties of X-rays 

 is their power of making gases a temporary con- 

 ductor of electricity. The study of this small con- 

 ductivity led to a clear idea of the transfer of 

 electricity through gases by means of charged 

 ions, and the nature and difference of the positive 

 and negative ions have been closely studied". The 

 proof by Townsend of the production of ions by 

 collision in electric fields opened up a new field 

 of investigation and gave us for the first time a 

 clear idea of the processes leading up to an electric 

 spark. The ionisation theory was found to explain 

 the conductivity produced by radium rays and the 

 conductivity of flames. The laws controlling the 

 escape of electricity from glowing bodies were 

 closely examined by H. A. Wilson and O. W. 

 Richardson. 



It is a striking fact that these purely scientific 

 researches on the conductivity of gases, which 

 had their inception in the Cavendish Laboratory, 

 and appeared at first to have only an aca- 

 demic interest, should so soon have resulted in 

 important practical applications. We may in- 

 stance the use of a hot filament in a low vacuum 

 as a rectifier of alternating currents and a detector 

 of electrical waves. The supply of electrons from 

 a glowing filament coupled with the generation 

 of ions by collision has led to the production of 

 powerful electric oscillators and amplifiers for 

 magnifying minute currents to any desired degree. 

 These amplifiers have not only been of great 

 service in war, but have also rendered possible 

 radiotelephony across the Atlantic. Last, but not 

 least, we have the invention of the Coolidge X-ray 

 tube, which has played such an important part in 

 research and in radiography. 



While the mechanism of ionisation of gases by 

 X-rays and radium rays and the transfer of elec- 

 tricity in ordinary electric fields is in the main 

 well understood, it is a striking fact that the 

 passage of the disruptive discharge through a 

 vacuum tube, which was the starting point of so 

 many discoveries, is still almost a mystery. While 

 no doubt some of the main factors involved in the 

 discharge are known, the phenomena in gases at 

 low pressure are so complex that we are still far 

 from a complete elucidation of the problem. This 

 complexity is well instanced, for example, by the 

 sign and magnitude of the charges communicated 

 to atoms and molecules in the positive rays, which 

 have been so closely studied by Wien and .Sir J. J. 

 Thomson, and in the hands of the latter have 

 given us a very delicate method of chemical 

 analysis of gases in a discharge tube. 



The discovery of the electron as a mobile con- 

 stituent of the atom of matter has exercised a 

 wide influence on electrical theory, and has been 

 the starting-point of attack on numerous electrical 

 problems. In these theories the electron may be 

 considered as a point charge with an appropriate 

 NO. 2610, VOL. 104] 



mass associated with it, and in many cases no 

 assumptions as to the nature and constitution of 

 the electron itself are involved. One of the first 

 problems to be attacked was the passage of elec- 

 tricity through metals where it was supposed that 

 the negative electrons are continuously liberated 

 from the atoms, and are in temperature equi- 

 librium with the matter. While the theories as 

 initially developed by Drude and Sir J. J. 

 Thomson have been instrumental in accounting 

 for a number of relationships, they are unsatis- 

 factory on the quantitative side. These difficulties 

 have been enhanced by the recent discoveries of 

 Kamerlingh Onnes of the supra-conductivity of 

 certain pure metals at very low temperatures and 

 the marked departure from the law of Ohm under 

 certain conditions. .As in the case of the theory 

 of radiation, it may be necessary for an ultimate 

 explanation to introduce the ideas of quanta as 

 recently proposed by Keesom. Langevin has 

 applied the electron theory to the explanation of 

 magnetism and diamagnetism, but there are still 

 many difficulties. The suggestion, first proposed 

 by Weiss, that there exists a natural unit of 

 magnetism called the magneton, analogous in 

 some respects to the atom of electricity, still lacks 

 definite confirmation. 



In this brief review reference can be made only 

 to the apparently insoluble difficulties in the ex- 

 planation of the facts of radiation brought to 

 light in recent years, and to the application of the 

 theory of quanta which has had such a large 

 measure of success in many directions. 



Radioactivity . 



The rapid growth of the subject of radio- 

 activity after the discovery by Becquerel of the 

 radiating power of uranium was greatly influ- 

 enced by the discovery and isolation of radium in 

 1899 by Mme. Curie, for the radioactive proper- 

 ties of this element were on such a scale of mag- 

 nitude that they were difficult to explain and still 

 more difficult to explain away. The systematic 

 chemical analysis of uranium ores disclosed the 

 presence of new radioactive substances like polo- 

 nium and actinium, while the study "of thorium^ 

 radium, and actinium disclosed the emission of 

 radioactive emanations or gases and their appar- 

 ently remarkable power of conferring temporary 

 activity on all bodies in their neighbourhood. The 

 changes in activity of these substances with time 

 and the different types of radiation emitted at first 

 gave an appearance of great complexity and con- 

 fusion to the rapidly accumulating mass of facts, 

 but the whole subject took on an orderly and 

 systematic development after the transformation 

 theory was put forward by Rutherford and Soddy 

 in 1903 as an explanation of radioactivity. On 

 this view radioactive matter is undergoing spon- 

 taneous transformation of its atoms with the 

 appearance of a succession of new radio- 

 active bodies, each marked by characteristic 

 chemical and radioactive properties. The 

 radiations accompany the transformation of 



