Page:Popular Science Monthly Volume 67.djvu/19

Rh activity varies, the activity measured by the $$\gamma$$ rays always varies in the same proportion. Active matter which does not emit $$\beta$$ rays does not give rise to $$\gamma$$ rays. For example, the radio tellurium of Marckwald, which does not emit $$\beta$$ rays, does not give off $$\gamma$$ rays.

Certain differences are observed, however, in the ionizing action of $$\gamma$$ and X rays. For example, gases and vapors like chlorine, sulphuretted 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 $$\gamma$$ rays follows the density law very closely. It seemed likely that this apparent difference between the two types of rays was due mainly to the greater penetrating power of the $$\gamma$$ rays. This was confirmed by some recent experiments of Eve, who found that the relative conductivity of gases exposed to very penetrating Röntgen rays from a hard tube approximated in most cases closely to that observed for the $$\gamma$$ 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 $$\gamma$$ rays.

Thus the results so far obtained generally support the view that the $$\gamma$$ 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 $$\gamma$$ rays will always appear with the $$\beta$$ rays.

No evidence of the emission of a type of Röntgen rays is observed from active bodies which emit only $$\alpha$$ 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 positive charge in its passage through matter.

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