94 PRESIDENT’S ADDRESS. 
subject for investigation; experiments on interference between rays of 
light emerging in different directions from the luminous source would 
probably throw light on this point. 
I now pass to a very brief consideration of one of the most important 
and interesting advances ever made in physics, and in which Canada, 
as the place of the labours of Professors Rutherford and Soddy, has 
taken a conspicuous part. I mean the discovery and investigation of 
radio-activity. | Radio-activity was brought to light by the Réntgen 
rays. One of the many remarkable properties of these rays is to excite 
phosphorescence in certain substances, including the salts of uranium, 
when they fall upon them. Since Réntgen rays produce phosphor- 
escence, it occurred to Becquerel to try whether phosphorescence would 
produce Réntgen rays. He took some uranium salts which had been 
made to phosphoresce by exposure, not to Réntgen rays but to sunlight, 
tested them, and found that they gave out rays possessing properties 
similar to Réntgen rays. Further investigation showed, however, 
that to get these rays it was not necessary to make the uranium phos- 
phoresce, that the salts were just as active if they had been kept in the 
dark. It thus appeared that the property was due to the metal and not 
to the phosphorescence, and that uranium and its compounds possessed 
the power of giving out rays which, like Réntgen rays, affect a photo- 
graphic plate, make certain minerals phosphoresce, and make gases 
through which they pass conductors of electricity. 
Niepce de Saint-Victor had observed some years before this discovery 
that paper soaked in a solution of uranium nitrate affected a photo- 
graphic plate, but the observation excited but little interest, The 
ground had not then been prepared, by the discovery of the Rontgen 
rays, for its reception, and it withered and was soon forgotten. 
Shortly after Becquerel’s discovery of uranium, Schmidt found 
that thorium possessed similar properties. 'Then Monsieur and Madame 
Curie, after a most difficult and laborious investigation, discovered two 
new substances, radium and polonium, possessing this property to an 
enormously greater extent than either thorium or uranium, and this 
was followed by the discovery of actinium by Debierne. Now the 
researches of Rutherford and others have led to the discovery of so many 
new radio-active substances that any attempts at christening seems to 
have been abandoned, and they are denoted, like policemen, by the 
letters of the alphabet. 
Mr. Campbell has recently found that potassium, though far inferior 
in this respect to any of the substances I have named, emits an appre- 
ciable amount of radiation, the amount depending only on the quantity 
of potassium, and being the same whatever the source from which the 
potassium is obtained or whatever the elements with which it may be 
in combination. 
The radiation emitted by these substances is of three types, known 
