UTAH ACADEMY OF SCIENCES 109 
The atomic theory still remains much as Dalton and 
his contemporaries left it, and nothing to cause any note- 
worthy revision of it came to light for many years after 
their day. It is true that certain chemists in Europe and 
in America turned against the atomic theory, and Ostwald 
even went so far as to write a textbook of chemistry in 
which the atomic theory was practically ignored. This 
movement, however, did not gain a very large following, 
more perhaps because of the inertia of the chemists than 
for any other reason. 
It was not until 1895 that a discovery was made 
which had an ultimate effect upon the ideas previously 
held regarding the composition of matter, and which 
finally occasioned a restating, or perhaps more properly 
an enlarging of the atomic theory. This was Roentgen’s 
discovery of the X-rays. 
It is not within the scope of this paper to take up in 
detail this discovery, and its consequences, but for the 
sake of completeness I will outline briefly the subsequent 
developments. The remarkable properties of the X rays 
led, of course, to a thorough investigation of their nature 
and of their source. This led to a closer examination 
of the cathode rays produced in a vacuum tube, since the 
two were apparently connected, and in 1897 J. J. Thomp- 
son showed that the cathode rays consisted of a stream of 
particles, of apparent mass about 0.001 that of the hydro- 
gen atom moving with great velocity. and carrying nega- 
tive charges of electricity. One of the peculiar properties 
of the cathode rays is to set up phosphorescence on the 
walls of the vacuum tube, and it was at one time consid- 
ered possible that this phosphorescence was in some way 
connected with the production of X rays. This we now 
know not to be the case, but the suggestion had most pe- 
culiar results. It occured to several investigators that, if 
the induced phosphorescence in the vacuum tube could set 
up X rays, then perhaps substances which are naturally 
phosphorescent give off something similar to X rays. One 
of these investigators was Becquerel, and among the sub- 
stances which he investigated was potassium uranium 
sulphate. This he put in a package with a photographic 
plate, but separated from the plate by several layers of 
black paper, and the whole was left for some time in a 
