UTAH ACADEMY OF SCIENCES 111 
rays consist of particles about four times the size of an 
atom of hydrogen, the beta rays consist of particles 
about 1/1800 the size of a hydrogen atom, while the gam- 
ma rays are probably of the nature of X rays, but of short- 
er length. The alpha rays have been shown to produce 
atoms of helium, the beta rays are, perhaps, atoms of 
negative electricity, electrons, and may be the ultimate 
particles of which all matter is composed. In addition to 
the above, the alpha rays are quickly absorbed by the 
‘atmosphere, the beta rays are able to penetrate thin metal- 
lic: foils, while the gamma rays penetrate considerable 
‘thickness of metals. 
“About 80 radio active substances have been recog- 
nized, and some of them have found their places in the 
‘Periodic Table of the Elements. Briefly stated, a radio- 
active element is one whose atom may become unstable, 
“and spontaneously break up. This may happen to many 
‘atoms in ‘a:short time, i. e., the decomposition is rapid, or 
it may happen to only a few in the same time. When 
“Such an atom decomposes, it may give off alpha particles, 
‘i e., helium atoms, ‘and it is changed into something else. 
‘This new substance may change again quickly into a 
‘third, and so on through many transformations, or, the 
new substance may be quite stable. On the other hand, 
if the atom, on decomposing, gives off beta particles and 
“not ‘alpha, its mass remains almost ‘unaltered, and its 
chemical and physical properties are’ perhaps unchanged. 
We have, then two'very important additions to the 
atomic theory made by the radioactive elements. In the 
‘first place the chemical atom is not the limit of divisibility. 
This place has now been taken by the’electrical atom, the 
‘electron. ‘In the second place the atoms of the’ chemical 
elements are not all’alike. The second point may best’be 
‘understood by following the course of ‘a radioactive ‘dis- 
integration, as shown graphically by the chart. It will be 
“seen that, beginning with uranium, successive radioactive 
disintegrations occur until finally the atomic weight has 
‘changed from 238 to 206, and the substance has changed 
from uranium into lead. During this transformation 
“several new forms of matter have appeared, some of them 
‘being very short lived, others relatively stable, and sdme 
of them undoubtedly fit into the periodic table of the 
