176 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



microscope will drift in a given time, and our own experiments 

 have checked this prediction to within one-half per cent. It is 

 that sort of evidence that has convinced Ostwald of the correctness 

 of the kinetic and the atomic theories. 



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 stagnant, but when a beam of X-rays is shot between the 

 plates some of the molecules become electrically charged and begin 

 to jump, a part of them toward the upper plate and a part toward the 

 lower plate, where their presence can be detected 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 20 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 occurred just 

 a little after the discovery of X rays. And here again we found 

 matter doing things we had never dreamed it w T as doing — viz., shoot- 

 ing 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 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 20 years ago. 



The fourth discover} 7 that I wish to mention is the discovery of 

 the atomicity of electricty, the proof that the thing we call electricity 

 is built up out of a definite number of specks of electricity, all exactly 

 alike, and that what we call an electrical current consists simply 

 in the journey along the conductor of these electrical specks, which 

 we may call with perfect justice definite material bodies. Now, I 

 can give you in just a word the proof of that statement. There are 

 half a dozen ways in which it could be approached. I will mention 

 the one with which I am most familiar, because it is the particular 

 proof which we worked out at our laboratory. We took these plates 

 with a field of 10,000 volts between them, with a little hole in the top 

 plate, and we blew an oil spray above the top plate so as to get an 

 electrically charged body just as small as we could, for we expected 

 that the frictional process involved in blowing the spray would 

 charge the drops, which it was found to do. We let one of these 

 drops come into the space between the plates and then moved it up 

 and down by an electrical field, throwing on the field as it came close 

 to the bottom plate, and throwing it off as it approached the upper 

 one, and so we kept that oil drop going up and down between the 



