42 PHOSPHORESCENCE 



These striking experiments were made some 

 few years ago by Professor Quet, and have ex- 

 cited general admiration wherever they have been 

 seen. Now it has occurred to M. Ed. Becquerel, 

 to enclose certain phosphorescent substances, such 

 as sulphide of barium, etc., in glass tubes, in 

 which a vacuum has been produced by the air- 

 pump, and to submit them in these circumstances 

 to the action of M. RuhmkorfPs apparatus. 



We have already seen that sulphide of barium, 

 of strontium, of calcium, diamonds, chalk, etc., 

 acquire phosphorescence when submitted to an 

 electric discharge in the air ; as soon as the dis- 

 charge has passed, they glow with phosphoric 

 light of short duration, just as if they had been 

 exposed to the sun, or as if they had been heated ; 

 for these substances are phosphorescent by light, 

 by heat, and by electricity. But when they are 

 submitted to the rapid series of discharges of the 

 induction apparatus in highly -rarefied air, that is, 

 in the void produced by the air-pump, the effect 

 is very striking. The substances named glow 

 continuously with a vivid phosphoric light, so 

 long as the discharges continue to pass.* 



In these experiments it has been observed, that 

 the glass of the tubes becomes slightly phospho- 

 rescent at the same time as the sulphides. 



Quet made known in 1853 a very curious pro- 

 * See Ed. Becquerel, Ann. de Chim. Iv. p. 92 et seq. 



