THE BECQUEREL RAYS 135 



be focussed to a point, if the cathode be given the form of 

 a concave mirror. Objects placed in the focus of such a 

 mirror are bombarded, according to Sir William, and may 

 be heated to whiteness by the impacts they receive from 

 the prodigious number of moving molecules. Goldstein, 

 on the other hand, conceived the phenomena to be due to 

 a transmission of energy, apart from the conveyance of 

 material particles ; but he gave no precise definition of the 

 nature of this transmitted energy. In 1883, however, 

 Professor Wiedemann of Leipzig made the suggestion 

 that possibly such ' cathode rays/ as the rectilinear dis- 

 charges have since been termed, are composed of radia- 

 tions of very short wave-length, shorter even than those 

 of the most ultra-violet light. The same conception was 

 held by Lenard. But while the cathode rays are deviated 

 by a magnet, light waves are uninfluenced ; and this forms 

 an argument in favour of the former being due to pro- 

 jected particles. The suggestion has also been made, but 

 on no sufficient grounds, that these phenomena are attri- 

 butable to a longitudinal vibration of the ether, the waves 

 being thus analogous to sound-waves in air alternate 

 condensations and rarefactions; or to choose a visible 

 analogy, the longitudinal vibrations of a spiral spring, in 

 which the coils periodically come closer together at one 

 point of space, and then recede and become wider apart. 

 A fourth hypothesis, similar to, yet differing from that of 

 Crookes, is held by Professor J. J. Thomson of the 

 Cavendish Laboratory, Cambridge. His view, which 

 appears to be well supported by experimental evidence, 

 is that each molecule of gas, in absorbing its electric 

 charge, dissociates, or splits up, into two or more charged 

 atoms or groups of atoms. Such charged portions of 

 matter have long been taken for granted as existing 

 during the passage of an electric current through a con- 

 ducting liquid, and were named by Faraday, ions, or 



