196 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
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 constituent of the atom 
of matter has exercised a wide influence on electrical theory, and 
has been the starting point of attack on numerous electrical prob- 
lems. In these theories the electron may be considered as a point 
charge with an appropriate 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 electricity through metals where it was supposed that 
the negative electrons are continuously liberated from the atoms, 
and are in temperature equilibrium 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 unsatisfactory on the quantitative side. These difficulties have 
been enhanced by the recent discoveries of Kamerlingh Onnes of 
the supraconductivity of certain pure metals at very low tempera- 
tures 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 explanation 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 radioactivity after the dis- 
covery by Becquerel of the radiating power of uranium was greatly 
influenced by the discovery and isolation of radium in 1899 by Mme. 
Curie, for the radioactive properties of this element were on such a 
