MIRACLES OF SCIENCE 



The test which showed the ultimate capacity of the 

 electroscope was made recently by Professor Ernest 

 Rutherford. He connected an electroscope with a 

 closed cavity having a small aperture on one side, 

 and near this aperture he placed a surface covered 

 with radium. A certain number of the alpha particles 

 thrown out by the radium could enter the receptacle 

 through the aperture. The radium was placed at 

 such a distance that only three or four particles per 

 minute, of the shower flying in all directions, would 

 be shot through the little window. 



It was found possible to adjust the electroscope to 

 such a state of delicate responsiveness that the en- 

 trance of a single alpha particle discharged it. 



Thus it was possible to compute the number of 

 alpha particles that are sent out by a given quantity 

 of radium in a given time. Other experiments con- 

 ducted by Professor James Dewar, of London, have 

 carefully measured the quantity of helium gas that 

 arises from a given quantity of radium. It is ob- 

 vious that the two experiments combined show the 

 number of helium atoms that make up a given quan- 

 tity of helium gas. 



Now it has long been known that all gases under 

 equal conditions of temperature and pressure, con- 

 tain the same number of molecules. A molecule 

 may contain one or more atoms, but this also is 

 something that the physicist has long been able to 

 compute. It follows that the physicist is now able, 

 thanks to the test performed by Professor Ruther- 

 ford with the electroscope, to compute the number 

 of atoms in any gas of known chemical composition. 



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