526 SCIENCE PROGRESS 



spontaneously, however; the substances producing them are 

 not radioactive. 



In 1896 Henri Becquerel found that potassium uranyl 

 sulphate emitted rays which acted on a photographic plate 

 enveloped in black paper, and even separated from the uranium 

 salt by thin plates of metal and other opaque substances. He 

 found later that all the compounds of uranium, and the metal 

 itself, possessed the same property. They not only appeared 

 capable of giving off these radiations continuously, thereby 

 giving rise to a continual supply of energy, but seemed perfectly 

 unchanged by so doing. The action is spontaneous, and 

 uranium is therefore the first element in which the property 

 of radioactivity was discovered. 



Uranium salts, besides acting on photographic plates, 

 possess the other radioactive property of discharging electrified 

 bodies. A charged electroscope consisting of strips of gold or 

 aluminium leaf suspended from an insulated support, and 

 separated from each other by the repulsion of their electric 

 charge, is usually employed to test materials supposed to be 

 radioactive. Not only is the electricity discharged by radio- 

 active materials, but the rate at which the leaves fall together 

 varies with the degree of radioactivity, other conditions being 

 the same. Different uranium salts discharge an electroscope at 

 different rates ; they have different activities. Madame Curie has 

 shown that this difference is due, primarily, to the varying 

 proportion of uranium ; the radioactivity is proportional to 

 the percentage of uranium in the salt employed. When she 

 extended these quantitative measurements to uranium minerals, 

 this relationship was found to hold no longer. Certain varieties 

 of pitchblende — which consists largely of uranium oxide — were 

 four times as radioactive as the uranium they contained. 

 Chalcolite, copper uranium phosphate, was twice as active ; 

 artificial chalcolite gave a figure agreeing with theory. These 

 differences led M. and Mme. Curie to analyse systematically 

 the mineral pitchblende, using the degree of activity as a 

 test for every precipitate, and separating by repeated opera- 

 tions the radioactive from the inactive precipitates. In this 

 way the two elements polonium and radium were isolated 

 in 1898. 



By similar methods during the last eleven years no fewer 

 than twenty-four new elements have been discovered, most of 



