CUMULATIVE EVIDENCE FOR EVOLUTION 



solar system, composed of a central relatively massive body 

 around which revolve one or more planetary bodies, each 

 occupying perhaps only about a millionth part of the space 

 occupied by the smallest atom. An atom of hydrogen is the 

 simplest atomic system known. It is composed of but one 

 proton, or positively charged central particle, and but one 

 electron, or negatively charged particle, which revolves in an 

 orbit about it. The speed of the revolution of the electron 

 is so tremendously great that it is practically everywhere at 

 once within its orbit. The distance between the proton and 

 the electron seems so enormous as compared with the minute 

 size of these particles that one is forced to the conclusion that 

 the inside of the atom is mainly empty space. One may get 

 a more concrete idea of the relative sizes and distances within 

 the atom by comparing the atom of hydrogen with the earth 

 and the sun. It may be said, speaking broadly, that if the 

 orbit of the electron about the proton in an atom of hydrogen 

 were enlarged to the size of the orbit of the earth about the 

 sun the electron would have a diameter about equal to that 

 of the sun and the proton a diameter about equal to that of 

 the earth. This somewhat topsy-turvy relation is due to the 

 fact that the proton, though ever so much smaller than the 

 electron, is nearly two thousand times as massive, or heavy. 

 Other atoms are far more complex than the hydrogen 

 atom, some of them containing over two hundred times as 

 many protons and electrons. No matter how large and com- 

 plex an atom becomes, it includes no other kinds of particles 

 than those contained in the simplest atom. All differences 

 in the properties of the elements are due to the number of 

 and the variations in the arrangement and configuration of 

 these ultimate particles. The nucleus of any other atom than 

 that of hydrogen is composed of both protons and electrons, 

 firmly organized into a relatively stable core, around which 



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