RADIOACTIVITY A FACTOR OF PLANT ENVIRONMENT 33 



activity a matter of especial interest and importance to the plant 

 physiologist.* 



Reasoning from various experiments, Thomson ^'^^ states that, *' a 

 radioactive substance, apparently radium, is exceedingly widely dis- 

 tributed, occurring in the most unexpected places," e. g:, soil, bricks, 

 glass, sea sand from Whitby beach, Yorks ("exceedingly rich"), 

 one specimen of wheat flour (others none), all clays, sands, and 

 gravels that were examined. Nearly all bodies, he states, emit radi- 

 ations which can ionize a gas. Experiments of Wood "^ lead to the 

 same conclusion, and Strutt "^' '^'^ has calculated that the amount of 

 radioactivity necessary to compensate for the radiation of heat by the 

 earth is much exceeded by the activity of ordinary materials. The 

 spontaneous ionization of the air in closed glass vessels may possibly 

 be partly accounted for by the fact that clay and other silicates are 

 known to emit large numbers of ions.^^ | 



In this connection may be recalled the paper by Eve,^® describing 

 the infection of the entire physics-building of McGill University with 

 radium. The use of the radium had been confined to one portion of 

 the building, but, later, objects from all parts were found to be 

 strongly radioactive. " Sheets of mica, lead foil, iron, zinc, and tin 

 were all active, even when taken from drawers and cupboards." 

 About 90 per cent, of the activity could be removed by solution in 

 strong hydrochloric acid, and the acid thereby became active. The 

 influence has spread from room to room, and from floor to floor in 

 the large building. 



Lenard^* has found that when light of short wave-length falls 

 on metal surfaces slowly moving ions are shot into space. Their 

 initial velocity varies, not with the intensity, but with the quality of 

 the light, and they ionize gases through which they pass, thus making 

 them conductors of electricity. 



The Effect of Radioactivity on Surroundings : In addition 

 to causing excited radioactivity, as noted above, radioactivity may 

 affect the plant's surroundings in other ways. Oxygen, for example, 

 is changed to ozone by the rays from radiferous barium. Further- 

 more a slight rise in temperature may result. Curie and Labord'^'" 

 were the first to announce the emission of heat from radium. One 



* Phillips ^1 has recently (28 May, 1908) reported evidence of an emanation from 

 sodium that will discharge an electroscope negatively (but not positively) charged. 

 Whether this is a true radioactive gas, or not, has not yet been demonstrated. 



tCf. also Wood, A.'*^ 



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