142 CHEMICAL DISCOVERY AND INVENTION 



emitted by reason of phosphorescence, since at the end of one 

 hundredth of a second phosphorescence becomes so feeble as to 

 be imperceptible. It appeared then that the photographic 

 cation produced by an uranium compound was a phenomenon 

 of a completely new order. In order to ascertain whether it was 

 due to energy stored up in the crystal and that the effect would 

 disappear or diminish with time, some years would have been 

 necessary. Fortunately Becquerel discovered almost immedi- 

 ately after the photographic experiments just mentioned, that 

 the new radiation had the property of rendering the surrounding 

 medium an electrical conductor, and consequently of dis- 

 charging electrified bodies when 

 brought near them. He used a 

 gold leaf electroscope, of which 

 a common form is shown in the 

 figure. A couple of strips of 

 gold leaf are attached to the end 

 of a brass rod having a brass 

 disc at the top. The leaves must 

 be protected from draughts of 

 air, etc., and they are therefore 

 suspended inside a glass case, the 

 rod being electrically insulated 

 by means of ebonite or some 

 good non-conductor. When the 

 plate is electrified the gold leaves 

 receive part of the charge, and 

 both being alike either positive 

 or negative are repelled by each 

 other and so diverge. Under 

 ordinary circumstances in air the charge slowly leaks away and 

 the leaves collapse again. In the presence of a radio-active sub- 

 stance the leaves of the electroscope collapse more rapidly, at 

 a rate which can be measured by having a scale on the glass case. 

 Like the X-rays those discovered by Becquerel travel in 

 straight lines and are capable of traversing wood, paper, and 

 the less dense metals such as aluminium. The radio-activity of 

 a uranium compound is, however, tested more conveniently by 

 the use of the electroscope, and it was by the systematic use of 

 this instrument that Madame Curie, assisted by her husband, 

 the late Professor Curie, was led to the discovery of radium. 



FIG. 52. GOLD LEAF ELECTROSCOPE. 



