PRESENT PROBLEMS IN RADIOACTIVITY. 13 



activity varies, the activity measured by the y rays always varies in 

 the same proportion. Active matter which does not emit /? rays does 

 not give rise to y rays. For example, the radio tellurium of Marck- 

 wald, which does not emit /? rays, does not give off y rays. 



Certain differences are observed, however, in the ionizing action of 

 y and X rays. For example, gases and vapors like chlorine, sulphuret- 

 ted hydrogen, methyl-iodide and chloroform, when exposed to ordinary 

 X rays, show a much greater ionization, compared with air, than is to 

 be expected according to the density law. On the other hand, the 

 relative ionization of these substances by y rays follows the density 

 law very closely. It seemed likely that this apparent difference be- 

 tween the two types of rays was due mainly to the greater penetrating 

 power of the y rays. This was confirmed by some recent experiments 

 of Eve, who found that the relative conductivity of gases exposed to 

 very penetrating Eontgen rays from a hard tube approximated in 

 most cases closely to that observed for the y rays. The vapor of 

 methyl-iodide was an exception, but the difference in this case would 

 probably disappear if X rays could be generated of the same penetrating 

 power as that of the y rays. 



Thus the results so far obtained generally support the view that 

 the y rays are a type of penetrating X rays. This view is in agreement 

 too with theory, for it is to be expected that very penetrating y rays 

 will always appear with the /? rays. 



No evidence of the emission of a type of Eontgen rays is observed 

 from active bodies which emit only a rays. If the a particles are 

 initially projected with a positive charge, such rays are to be expected. 

 Their absence supplies another piece of evidence in support of the view 

 that the a particle is projected without a charge but acquires a posi- 

 tive charge in its passage through matter.* 



Emission of Energy by the Radioactive Bodies. 



It was early recognized that a very active substance like radium 

 emitted energy at a rapid rate, but the amount of this energy was 

 strikingly shown by the direct measurements of its heating effect made 

 by Curie and Laborde. They found that one gram of radium in 

 radioactive equilibrium emitted about 100 gram calories of heat per 

 hour. A gram of radium would thus emit 876,000 gram calories per 

 year, or over 200 times as much heat as is liberated by the explosion 

 of hydrogen and oxygen to form one gram of water. They showed 

 that the rate of heat emission was the same in solution as in the solid 

 state, and remained constant when once the radium had reached a 

 stage of radioactive equilibrium. Curie and Dewar showed that the 



* Recent experiments indicate, however, that the a particles are charged at 

 the moment of this expulsion. 



