32 RADIOACTIVITY A FACTOR OF PLANT ENVIRONMENT 



all substances." The larger portion of the rays, according to Camp- 

 bell, are a rays. 



During the course of experiments at the foot of Niagara Falls, 

 McLennan^ found that three stretches of No. 24, insulated copper 

 wire, of about 30 meters each, exposed to the spray, immediately be- 

 came negatively charged to a potential of about 7'5*^o volts. The 

 charge was shown to be caused by the spray, and to vary with its 

 density. The spray also excited radioactivity in the air, but when 

 evaporated it left no radioactive residue. 



From the fact that air bubbled through distilled water in which 

 lead acetate or lead nitrate has been dissolved is more radioactive 

 than when bubbled through pure water, Thomson^-' infers that lead 

 is radioactive. Brick,'^" metals generally,'^* the minerals samarskite, 

 pitchblende, and monazite, from North Carolina, Cornwall, Norway, 

 and Brazil,"' and tin, zinc, graphite, platinum, lead, aluminum, and 

 carbon, -•'' all manifest radioactivity. Munoz '^ has studied minerals, 

 earth, water, and gases of the atmosphere in various parts of Spain, 

 and finds radioactivity widely distributed throughout that country. 



" From the kinetic theory of gases," says Lord Kelvin,''^ *' it 

 seems certain that every kind of matter has some radioactivity ; that 

 is to say, shoots off both vitreously and resinously electrified particles. 

 Hence it is only in their extraordinarily great abundance and great 

 velocities of shooting, that polonium and radium differ from ordi- 

 nary matter." * 



Campbell -" concludes from his experiments that potassium and 

 rubidium are radioactive substances. The rubidium is less active 

 than potassium, while the activity of the latter is yoVo *^^* ^^ 

 uranium. The rays given off by pptassium are /9 rays. Tests with 

 sodium gave negative results.! The radioactivity of potassium has 

 been confirmed by McLennan.^*"' The fact that potassium is an 

 essential constituent of the food of most plants renders its radio- 



* In the same paper Lord Kelvin describes a model of an atom to illustrate the 

 mode by which, according to his idea, the a and ,3 particles are given off. 



t In 1907 Professor J. J. Thomson ^^' succeeded in getting electrification from both 

 heating and rubbing various salts (phosphates and oxides), and explains the result as 

 due to the hypothetical fact that the salts are covered superficially with a double layer 

 of electrification, one layer negative, the 'other positive. The heating or rubbing 

 removes the outer laver and leaves the inner one unbalanced. He suggests that the 

 electrifving of bodies by friction results from the removal of one or both layers of 

 electrification by the rubbing. 



