90 THE REALITIES OF MODERN SCIENCE 



must assume, for of the repulsion he has positive 

 proof. He, therefore, postulates a medium which he 

 calls the "aether," or, as now spelled, "ether." The 

 word, derived from the Greek "belonging to the 

 upper air," was originally applied to the medium be- 

 tween the earth and the heavenly bodies. The ether 

 is imponderable and intangible and we cannot detect 

 its existence by our senses. 



When the physicist speaks of a vacuum he means a 

 space free from matter, but through it electrical repul- 

 sions are exerted and light, heat, and radio-telegraph 

 waves may pass. Through such ethereal spaces the 

 light of the sun and the stars reaches our earth. To 

 the modern physicist, these vacua are empty only as 

 far as concerns matter, that is, molecules, atoms, and 

 dislodged electrons. According to his ideas a vacuum 

 is full of ether. In fact, he considers all space through- 

 out the universe to be filled continuously by this ether, 

 like an enormous ocean, in which exist as specks the 

 electrons and the atoms which they form. That the 

 electrons are really mere specks in this universe of 

 ether we realize readily from the statement that the 

 radius of an electron is probably not larger than 2 X 10~ 13 

 cm. and that of a hydrogen atom is about 2X1CT 8 cm. 

 The whole atom is perhaps 100,000 times as large in 

 diameter as the electron. In other words, the radius of 

 the electron is about as large, compared to the radius 

 of the atom, as is the radius of our earth as compared 

 to the radius of the orbit in which it travels around 

 the sun. 



We may consider the size of our solar system to be 

 as large as the orbit traveled by the most remote planet. 



