198 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1919. 
small fraction in the side branch. It is quite probable that further 
study may lead to the discovery of a number of such dual transforma- 
tions. In the violent cataclysm that must accompany the transforma- 
tion of an atom, it is not unexpected that the constituents of the 
residual atom may arrange themselves in more than one configuration 
of temporary equilibrium. 
Much attention has been directed to the properties of the radium 
emanation — the radioactive gas constantly produced by the trans- 
formation of radium atoms. The equilibrium volume of this gas 
from 1 gram of pure radium is only six-tenths of a cubic millimeter, 
but contributes more than three-fourths of the total activit}?^ of 
radium. By concentration of purified emanation into fine glass tubes, 
very powerful sources of radiation have been obtained, which have 
proved of great utility both in the laboratory and for therapeutic 
purposes. Although only about one-tenth of a cubic millimeter of 
purified radium emanation has ordinarily been available for experi- 
ments, methods have been devised to determine its spectrum, mo- 
lecular weight, freezing and boiling points. 
We owe to Hahn the discovery of two fairly long-lived products of 
thorium called mesothorium and radiothorium. The mesothorium, 
which is separated with the radium from ores containing both 
thorium and uranium, is transformed into radiothorium. These 
products can be obtained of activity greater than radium for equal 
weights, and give us another source of powerful radiation. 
The discovery of the production of helium from radium by Eamsay 
and Soddy was of great importance in emphasizing the reality of the 
transformations occurring in radium. Eutherford showed that the 
a-rays which are shot out from radium consist of positively charged 
atoms of helium, so that all radioactive substances which emit a-rays 
give rise to helium. The production of helium by radioactive sub- 
stances explains the occurrence of large quantities of helium in 
uranium and thorium minerals, and indeed the prediction by Euther- 
ford and Soddy that helium would prove to be a product of radio- 
active transformation was based in part on this fact. 
The great majority of radioactive substances are transformed 
with the expulsion of helium atoms with great velocity, but in a few 
cases swift electrons appear. The appearance of helium in so many 
changes, coupled with the observation that many of the atomic 
weights of many known elements differ by four units — the atomic 
weight of helium — indicates that helium must be one of the secondary 
units of which many of the ordinary elements are built up. It is 
noteworthy that so far no definite evidence has been obtained that 
hydrogen is a direct product of radioactive transformation, although 
its complete absence would be very surprising. 
