MIRACLES OF SCIENCE 



particles could be crowded into the space of a cubic 

 centimeter. 



But Professor Rutherford's census of the atoms, 

 as just outlined, shows us that twenty billion times 

 that number of helium atoms would exist in the form 

 of gas in the same space. Of course the molecules 

 of a gas are widely separated. So it follows that the 

 smallest particle of solid matter visible through the 

 most powerful microscope contains many times 

 twenty, billion atoms. 



PHOTOGRAPHING THE ATOM 



In 1910 Sir J. J. Thomson discovered yet another 

 method of making individual atoms give visible evi- 

 dence of their presence. The medium through which 

 the record is transcribed is in this case the photo- 

 graphic plate. In a word, Professor Thomson liter- 

 ally photographs the atoms. His method of letting 

 the atom transcribe its own record on the sensitive 

 plate is by far the most delicate method yet devised 

 of analyzing the constituents of a gas. 



The gases to be tested are introduced in exceed- 

 ingly small quantities into a glass bulb which is 

 called a vacuum bulb because it is supposed to con- 

 tain nothing at all. When an electric current is 

 passed through this vacuum, the bulb glows with a 

 peculiar phosphorescence, and the now familiar 

 phenomena of the cathode ray are manifested. The 

 cathode ray, as we have seen, consists of negative 

 particles of electricity. It has now been shown that 

 particles of another type traverse the tube in the 

 opposite direction to that in which the cathode par- 



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