184 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



air outside for a hundred million years. Perhaps the most striking 

 illustration is as follows: Take a tumbler of water and— supposing 

 it possible — label all the molecules in it. Throw the water into the 

 sea, or indeed, anywhere you please, and after a period of time so 

 great that all the water on the earth — in sea, lakes, rivers, and 

 clouds — has had time to become perfectly mixed, fill your tumbler 

 again at the nearest tap. How many of the labeled molecules are to 

 be expected in it ? The answer is, roughly, 2,000 ; for although the 



Caesium 



26'" 

 Arrangement of Lead Atoms 



Lead 



Carbon 

 Comparative 

 Sizes of Atoms 



X %S. 



-So/ <s- 



Si 





Atoms in Quartz Crystals Si t 



i I I 1 I I I ! ■ I I I I I I II II 1 ! II I L 



' ' » ' I I ■ ' I 



35 uu 



Fig. 3. — Cube 26 showing atoms with scale of reference. 



number of tumblers full of water on the earth is 5 by 10 21 , the number 

 of molecules of water in a single tumbler is 10 25 . 



From the above statements it would, at first sight, appear absurd 

 to hope to obtain effects from single atoms, yet this can now be done 

 in several ways, and, indeed, it is largely due to the results of such ex- 

 periments that the figures can be stated with so much confidence. 

 Detection of an individual is only feasible in the case of an atom 

 moving with an enormous velocitj^ when, although its mass is so 

 minute, its energy is quite appreciable. The charged helium atom 

 shot out by radioactive substances in the form of an alpha ray pos- 

 sesses so much energy that the splash of light caused by its impact 



