246 THE HISTORY OF SCIENCE 



science and lead to results that no devotee of facts 

 and scorner of mere theory can well ignore. 



In 1895 Sir William Ramsay, who in the previous 

 year had discovered an inert gas, argon, in the at- 

 mosphere, identified a second inert gas (obtained 

 from minerals containing uranium and thorium) as 

 helium (^fXto?, sun), an element previously revealed 

 by spectrum analysis as a constituent of the sun. In 

 the same year Rontgen, while experimenting with the 

 rays that stream from the cathode in a vacuum tube, 

 discovered new rays (which he called X-rays) pos- 

 sessed of wonderful photographic power. At the be- 

 ginning of 1896 Henri Becquerel, experimenting on 

 the supposition, or hypothesis, that the emission of 

 rays was associated with phosphorescence, tested the 

 photographic effects of a number of phosphorescent 

 substances. He exposed, among other compounds, 

 crystals of the double sulphate of uranium and po- 

 tassium to sunlight and then placed upon the crystals 

 a photographic plate wrapped in two thicknesses of 

 heavy black paper. The outline of the phosphorescent 

 substance was developed on the plate. An image of a 

 coin was obtained by placing it between uranic salts 

 and a photographic plate. Two or three days after 

 reporting this result Becquerel chanced (the sunlight 

 at the time seeming to him too intermittent for ex- 

 perimentation) to put away in the same drawer, and in 

 juxtaposition, a photographic plate and these phos- 

 phorescent salts. To his surprise he obtained a clear 

 image when the plate w r as developed. He now assumed 

 the existence of invisible rays similar to X-rays. 

 They proved capable of passing through sheets of 

 aluminum and of copper, and of discharging electri- 



