CONDUCTION THROUGH GASES 137 



In the year 1895, Professor Rontgen of Munich 

 made the first of the sensational discoveries in 

 physical science for which the last thirty years 

 have been remarkable. Many other recent in- 

 vestigations have been as interesting, and several 

 have more profoundly modified our outlook on 

 Nature, but few have struck so readily the 

 imagination of the plain man as the revelation 

 of the skeleton within the living flesh. 



The origin of this discovery may be said to 

 have been almost accidental. Rontgen noticed 

 that photographic plates, kept under cover in 

 the neighbourhood of a highly exhausted tube 

 through which electric discharges were passing, 

 became fogged, as though they had been exposed 

 to light. He investigated this effect, and found 

 that, when cathode rays impinged either on the 

 glass of the tube, or on the anode, or on any 

 metallic plate within the tube, a type of radiation 

 was produced which would penetrate many sub- 

 stances opaque to ordinary light. Dense bodies, 

 like metal or bone, absorbed the rays more fully 

 than did lighter materials, such as leather or 

 flesh, and Rontgen, at once putting this discovery 

 to some purpose, was able to photograph the 

 coins in his purse and the bones in his hand. 



Given the rays, the mechanical contrivances 

 required to demonstrate their effects are not 

 elaborate. Rontgen rays produce phosphor- 

 escence on screens of barium platino-cyanide and 

 other similar salts, and, by using these screens 

 in place of a photographic plate, objects, usually 

 hidden from our eyes, may be made visible. 



A remarkable property of the rays is their 

 power of converting the air and other gases 



