14 CONTEMPORARY SCIENCE 



tion and counting of gaseous ions on the one hand . . . 

 and on the other the agreement of the Brownian move- 

 ments with the kinetic hypothesis . . . justify the most 

 cautious scientist in now speaking of the experimental 

 proof of the atomic theory of matter. The atomic hy- 

 pothesis is thus raised to the position of scientifically well- 

 founded theory." 



The second advance is the proof of the divisibility of 

 the atom, a proof which grew out of the discovery^ of 

 X-rays. Let me tell you how. If you have here two 

 plates with an electric field between them, and nothing 

 else but a monatomic gas like helium, then it is found that 

 when the field is thrown on, the helium is perfectly stag- 

 nant ; but when a beam of X-rays is shot between the 

 plates, some of the molecules become electrically charged 

 and begin to jump, some toward the upper, and some 

 toward the lower plate, where their presence can be de- 

 tected by an electrical measuring instrument. What does 

 that show? It shows that the thing which we call an 

 atom has electrical charges as its constituents ; and the 

 history of the last twenty years in physics has consisted 

 pretty largely in determining what are the properties of 

 these electrical constituents. 



The third is the discovery of radio-activity, which oc- 

 curred just a little after the discovery of X-rays. And 

 here again we found matter doing things we had never 

 dreamed it was doing; vis, shooting off from itself both 

 negatively and positively charged particles, the negatives 

 with a speed which may approach close to the velocity of 

 light, 186,000 miles per second, and the positives with a 

 speed of one-tenth of that, or 18,000 miles. The fact that 

 such speeds could be imparted to projectiles of any kind 

 was undreamed of twenty years ago. 



The fourth discovery that I wish to mention is the dis- 

 covery of the atomicity of electricity, the proof that the 

 thing we call electricity is built up out of a definite num- 

 ber of specks of electricity, all exactly alike ; and that 



