314 THE REALITIES OF MODERN SCIENCE 



All three types of rays produce ionization. The 

 rays themselves may be separated by subjecting them 

 to a transverse electrical field, for they consist of 

 streams of positive and negative corpuscles, which 

 will be oppositely deflected, and a true radiation which 

 suffers no deviation. Their ionizing effects are easily 

 traced in a gas containing water vapor by the phenome- 

 non of condensation about the newly formed ions. 

 The paths may therefore be photographed. Fig. 39 

 illustrates the case of a particles projected from radium 

 through air and Fig. 40 is an enlargement of a portion 

 of the preceding figure. Fig. 41 shows the path of a 

 13 particle. 



The paths shown by the a particles are from 3 to 7 

 cm. long. Now in each cubic centimeter of air there 

 are about 2.7 X10 19 molecules, so that each of the 

 particles traced by the photograph must have ionized 

 millions of air molecules. Through the electronic 

 systems which form these molecules each a particle 

 passed so rapidly as not to be deflected although it 

 repeatedly knocked off electrons. In two cases in Fig. 40 

 there may be seen sharp changes in the direction, 

 when one probably collided head-on with the massive 

 nucleus of an air molecule. These paths would in- 

 dicate that the nucleus of a molecule is relatively 

 very small indeed, for otherwise, in all probability, 

 there would have been many more such collisions. 



In Fig. 41, from bottom to top, we trace the ionization 

 of a high speed electron (/3 particle). It does not 

 ionize as frequently but it also follows a straight line. 

 Its velocity is obviously so high that it does not re- 

 main long enough within the neighborhood of the 



