204 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
this development were the Laws of Chemical Proportion as discovered 
and enunciated by Dalton near the beginning of the nineteenth cen- 
tury, and later the successes of the Kinetic Theory of Gases. By the 
beginning of the twentieth century, the concept of the chemical atom 
had received general acceptance as a theory based on scientific experi- 
mentation. The idea of atoms had thus been removed from the realm 
of philosophical speculation and had become a proved scientific fact. 
According to this picture all matter depending upon its nature consists 
of a mixture of varying numbers of the ninety-odd different chemical 
atoms. The size and the mass and other properties of most of the 
chemical atoms had been determined although not with great preci- 
sion. 
DISCOVERY OF FIRST ELEMENTARY PARTICLES 
During the time when the chemical atom was being firmly estab- 
lished as a scientific fact, other scientific investigations were succeed- 
ing in proving the existence of at least one particle of matter which 
was more elementary in character than the chemical atoms. In the 
decade from 1890 to 1900 the discovery of X-rays and radioactivity, 
and studies of the phenomena associated with the discharge of elec- 
tricity through gases, soon proved the existence of the electron and 
showed that the atoms of chemistry must all be considered as complex 
structures, structures which are themselves built up of particles of a 
more elementary character. 
The electron was distinguished from the other particles previously 
studied by physicists and chemists in one very important respect. It 
was established as a unique particle in the sense that all electrons 
were found to be identical with one another, no matter from what form 
of matter they were derived. For the first time then the presence of 
a particle truly elementary in character was revealed to science. It 
was found always to carry a negative electric charge and to have a 
mass about 2,000 times less than the hydrogen atom, the simplest 
and least massive of all the chemical atoms. The electron immedi- 
ately took its place as one of the elementary particles common to all 
forms of matter. 
The following 30 years, from 1900 to 1930, were extremely fruitful 
in furthering our knowledge of the properties of the chemical atoms. 
The work of Moseley showed that chemical atoms were members of 
a family, all of them being related to one another in a perfectly definite 
and simple way. In 1911 the experimental genius of Rutherford in 
Cambridge, England, proved the existence of the atomic nucleus, and 
in 1919 he succeeded for the first time in producing an atom of oxygen 
from the disruption of the nucleus of an atom of nitrogen. Thus in 
1919 the will of man for the first time was able to cause the disinte- 
gration of an ordinarily stable element, with the accompanying release 
