110 TRANSACTIONS OF. THE 
dark place. On developing the plate, it was found to be 
darkened, showing that the potassium uranium sulphate 
was giving off rays more penetrating than light. He soon 
discovered, however, that the actinic effect was not due 
to phosphorescence, but was a property of any uranium 
compound, or of metallic uranium itself, irrespective of 
the phosphorescence of the substance. Becquerel also 
found that the rays given off by uranium or its compounds 
‘had the property of discharging an electrified body. This 
property was examined by Rutherford, who showed that 
-the uranium rays were of two sorts, one much more pene- 
«trating than the other. ; 
Shortly after Becquerel’s discovery, Mme. Curie made 
a systematic examination of substances for radioactivity, 
as the; phenomena’ shown by uranium compounds were 
now called, and found that thorium possessed the similar 
property, and in. about the same degree as uranium. She 
also found that the radioactivity was a property of. the 
uranium or of the thorium, 2nd was not influenced by the 
other substances present. Soon after this she began an 
examination of uranium. and thorium minerals, and found 
.some. which were several times more active than urani- 
um itself. This could only be explained by assuming the 
presence of .an. unknown substance much more radio- 
active than uranium. She set to work then. to separate 
chemically, if possible, this substance, and actually separ- 
ated not one but two very active substances. The one 
which separated with bismuth she named polonium, and 
the one which separated with barium she named radium. 
So much, then, for the historical development. lead- 
ing up to the discovery of radioactivity, and of radio- 
active. substances. The subsequent development would 
be too time consuming to discuss historically and in detail, 
and I shall therefore briefly summarize our present know- 
ledge of the radio chemistry. -When the emanations from 
radioactive substances had been carefully examined it 
was found that they are of three types, called for con- 
venience the:.alpha, the beta and the gamma radiations. 
Any: radioactive.element may give off one or all of these. 
‘They are distinguished by the following properties :—the 
_alpha rays. are positively charged, the beta rays negatively 
charged, the gamma rays carry no charge. The alpha 
