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 complex an atom becomes, it includes no other kinds of particles than those contained in the simplest atom. All differences in the properties of 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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