EFFECTS OF THE RAYS OF RADIUM ON 



PLANTS 



CHAPTER I 

 THE DISCOVERY AND NATURE OF RADIOACTIVITY 



The Discovery of Cathode Rays : The discovery of radio- 

 activity was dimly foreshadowed as far back as the year 1838, when 

 Michael Faraday, ^^ studying the discharge of electricity through rare 

 gases, noted the fact that there was always a dark space between the 

 glows surrounding the positive and negative poles in the vacuum tube. 

 Fourteen years later Grove ^''observed and described the stratified ap- 

 pearance of the electric discharge through very rare gases. Gassiot ^* 

 further studied the stratification, and found that there were two dis- 

 tinct forms of stratified discharge, and that they could be deflected 

 and thus separated by a magnet. He *' later described the negative 

 discharge in a vacuum tube, and ascertained, not only that it could be 

 deflected by a magnet, but that, wherever the charge impinged, " a 

 brilliant blue phosphorescent spot is perceivable, which spot is in a 

 short time sensibly heated." Gassiot concluded that there is the ap- 

 pearance of " a direction of a force emanating from the negative," and 

 inferred from his experiments that an electric current cannot pass 

 through a perfect vacuum; the intervention, he said, of a certain 

 amount of matter is necessary. Four years later he ^^ published the 

 results of experiments confirming these conclusions, and stated that 

 there was " an actual disruption of particles from the negative termi- 

 nal," which indicates force there. 



The idea of cathode rays was more fully and accurately expressed 

 by Hittorf,^^ whose paper, *' Ueher die Electricitdts-Leitung der 

 Gase,'"\i^2irs the date of October 9, 1868, and was published in 1869, 

 six years after Gassiot's announcement. 



Lord Kelvin's*^^ statement* that the " kathode torrent" was dis- 



* Made also by Rutherford, 'i^ p. 73. 

 2 I 



