WORK OF HENRI BECQUEREL BROCA. 779 



and the apparatus was left in the dark. Later the pLite was de- 

 veloped and the impression was found as strong as if the sunlight 

 had struck the salt. Upon trying the experiment again Avithout 

 sunlight the same result was reached as if the sun had been used. 

 Although this uranium compound, which had been prepared some 

 time, was now kept in the darkness in a lead box, yet it still continued 

 to give the same results. 



The discharge of an electrified body under the influence of the 

 uranium emanation was next tried. This was at that time the only 

 process known which would give quantitative measures of this strange 

 ])ower. Then it was necessary to see if the phosphorescent state was 

 necessary for the newly found emanation. A nitrate of uranium 

 crystal, whether in solution or melted in its water of crystallization, 

 gave the same effect as when in the solid state, although in neither of 

 the liquid states would it phosphoresce. An attempt to see whether 

 bodies near such active compounds became active by a phenomenon 

 analogous to phosphorescence was unsuccessful. It was several years 

 later that the power of radium enabled M. and Mme. Curie to show 

 this and the profound difference between this new phenomenon and 

 luminous phosphorescence. 



Some odd results, not yet understood, led Becquerel for a moment 

 to erroneously believe that the new rays were ordinary radiation. 

 But he soon saw his error, noting that the propagation of this new 

 emanation took place as well across pulverized matter as across solid, 

 continuous bodies. 



Since all the compounds of uranium, whatever their chemical or 

 physical state, showed these phenomena, it was therefore natural to 

 attribute them to the uranium itself. Pure uranium was tried and 

 gave more intense results than the salts. It was now made evident 

 that neighboring bodies became the source of a secondary emanation 

 as long as they were struck by the uranium discharge, but that the 

 phenomenon ceased as soon as the body was removed from the 

 presence of the uranium. 



By pushing the experiments with the electrical discharge still 

 further it was shown that the air is rendered conductive and remains 

 so a few moments and that this plays an essential role in the phe- 

 nomenon. Air, active under the influence of the uranium and blown 

 upon an electroscope actively discharges the latter. If ordinary, 

 inactive air is bloAvn between the uranium and the ball of the electro- 

 scope, the latter is discharged more slowly. The emission seems 

 independent of the temperature of the uranium. The temperature of 

 the gas, however, modifies the discharge. 



In order to regidate this method of measuring the emanation, it 

 was necessary to find some law governing the discharge under the 

 varying potential. Becquerel established a limit of the velocity of 



