194 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
of ultra-violet light on metals and copiously from glowing bodies, 
while they were ejected with high speed spontaneously from the 
radioactive bodies. 
The interpretation by Lorentz of the Zeeman effect in which the 
spectrum lines were displaced by placing the source of light in a 
magnetic field showed that electrons of the same small mass were 
present in all atoms, and that their vibrations constituted visible 
light. Sir J. J. Thomson early pointed out the significance of the 
electron as one of the units of atomic structure and its importance 
in the mechanism of ionization in gases, and the rapid growth and 
acceptance of electronic ideas owes much to his work and teaching. 
An important stage in advance was the proof by Kaufmann that 
the mass of the electron was entirely electrical in origin. Sir J. J. 
Thomson had shown in 1881 that a charged particle acquired addi- 
tional or electrical mass in virtue of its motion. The variation of 
mass with speed has been shown to be in accord with general theory, 
but is in best agreement with the formula based on the theory of 
relativity. It would be of great interest to compare theory with 
experiment for the highest attainable speed of the electron from 
radium which is so near to the velocity of light that the variation 
of mass with velocity is very rapid. 
The proof that the electron was a disembodied atom of negative 
electricity was a great step in advance in electrical ideas. Informa- 
tion as to the nature of positive electricity is far less precise and 
definite, for no positive electron, the counterpart in mass of the 
negative electron, has ever been observed. In all experiments with 
positive rays and with radioactive transformations where the proc- 
esses are very fundamental in character, no positive charge has ever 
been found associated with a mass less than that of the atom of 
hydrogen. While it is. well to keep an open mind on this funda- 
mental question, the evidence as a whole suggests that there is an 
essential difference in mass between the carriers of positive and nega- 
tive electricity. In fact, such a difference seems to be essential to 
fit in with our knowledge of the structure of atoms. The nucleus of 
the lighest atom, hydrogen, may prove to be the positive electron 
and its much greater mass than that of the negative electron would 
then be ascribed to the greater concentration of the electrical charge 
in the former. 
From consideration of the passage of electricity through gases, 
it had long been surmised that electricity, like matter, was atomic 
in character. The study of the deflection of the cathode rays and 
a-rays in magnetic and electric fields showed that the carriers 
of each type had all the same charge, and the atomic nature of 
electricity was implicitly assumed by all workers. ‘Townsend showed 
