One of the great contributions of the nineteenth century to physics was the development of the idea of the luminiferons ether. The necessity of a medium for the transmission of the waves which produce the sensation of light had been recognized for many years and was clearly stated by Fresnel and others. Faraday had also conic to the conclusion that a medium was necessary for the transmission, or rather existence, of eleC tric and magnetic forces, and had suggested that possibly this medium might he identical with the one that carried 'light waves.' Alaxwell showed by theoretical considerations that the velocity with which an electromagnetic disturbance would he propagated in the medimn referred to by Faraday must be numerically equal to the so called 'ratio of the electrical units,' and further showed from experimental evidence that this was equal to the number expressing the velocity of light. De also proved by a mathematical in vestigation that the electromagnetic waves were identical in all respects with those with which we are familiar in the phenomena of light, and thus established the fact that only one medium is necessary for the explanation of the phe nomena of light, electricity, and magnetism. The connection between this ether and ordinary mat ter has been investigated by many observers, as described in the article on ETHER. The ac cepted theory at the present time is that the ether does not move as material bodies are car ried through it; and this, in connection with the ideas of electrons. has been elaborated by Lorentz into a theory which is able to explain in the most satisfactory manner the phenomena of dis persion, metallic reflection, and stellar aberra tion. There are many difficulties, however. which
still remain to be investigated in regard to the properties of the ether, in spite of the great prog ress marking the beginning of the twentieth cen tury. if the inertia of matter is to he regarded as capable of explanation, as indicated above, from the standpoint of electricity, there still re main two great problems for solution. What is the nature of an electric charge? and what is the cause of gravitation? The development of phys ics at the present time is tending toward investi gations of these two questions.
BIBLIOGRAPHY. Works: Winkelmann, Bibliography. Works: Winkelmann, Handbuch. der Physik (Breslau, 1891-96) ; Da niell, Principles of Pkysics (London and Yew York, 1894) Chwolson. Lehrinteh der Physik (Brunswick, 1902 et seq.) ; buch. der Physik and lleteorologie (91.1i ed., Brunswick, 1902 et seq.) : Ames, Theory of Phy sics (New York, 1897) ; ('ours de sique (Paris, 1890 Poynting and Thomson. Text-Book of Physics (London, 1899) ; Watson, Text-Book of Pkysics (ib., 1896).
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