All this leads to one inevitable conclusion: matter is electrical in nature. But now if matter and light have the same origin, and matter is subject to gravitation, why not light also? So reasoned Einstein. Summary. Newton's studies of matter in motion led to his theory of gravitation, and, incidentally, to his conception of time and space as definite entities. As we shall see, Einstein in his theory of gravitation based it upon discoveries belonging to the post-Newtonian period. One of these is Minkowski's theory of time and space as one and inseparable. This theory we shall discuss at some length in the next chapter. Other important discoveries which led up to Einstein's work are the researches which culminated in the electron theory of matter. The origin of this theory may be traced to studies dealing with the nature of light. Here again Newton appears as a pioneer. Newton's corpuscular theory, however, proved wholly untenable when Foucault showed that the velocity of light in water is less than in air, which is the very reverse of what the corpuscular theory demands, but which does agree with Huyghens' wave theory. But Huyghens' wave theory postulated some medium in which the waves can act. To this medium the name "ether" was given. However, all attempts to show the presence of such an ether failed. Naturally enough, some began to doubt the existence of an ether altogether. Huyghens' wave theory received a new lease of life with Maxwell's discovery that light is an electromagnetic phenomenon; that the waves set up by a source of light were comparable to waves set up by an electrical disturbance. Zeeman next showed that magnetism was also, closely related to light. A study of Zeeman's experiments led Lorentz to the conclusion that electrical phenomena are due to the motion of charged particles called "electrons," and that the vibrations of these electrons give rise to light. The conception of the electron as the very fundamental of matter was arrived at in an entirely different way: from studies dealing with the discharge of electricity through gases and the breaking up of the atoms of radium. If matter and light have the same origin, and if matter is subject to gravitation, why not light also? REFERENCES For the general subject of light the reader must be referred to a rather technical work, but one of the best in the English language: Edwin Edser, Light for Students (Macmillan, 1907). The nature of matter and electricity is excellently discussed in several books of a popular variety. The very best and most complete of its kind that has come to the author's attention is Comstock and Troland's The Nature of Matter and Electricity (D. Van Nostrand Co., 1919). Two other very readable books are Soddy's Matter and Energy (Henry Holt and Co.) and Crehore's The Mystery of Matter and Energy (D. Van Nostrand Co., 1917). III EINSTEIN