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THE POPULAR SCIENCE MONTHLY.

a popular work, the matter is presented in a manner to be of interest to others than professed students of the science, and should be read by those who desire to know something of the methods and appliances of experimental research in this branch of knowledge.

The subject is considered under the four general divisions of electro-statics, magnetism, electro-kinetics, and electro-optics. Of the first division, a large portion is devoted to the researches of various investigators, including the author, upon the specific inductive capacity of different insulating substances. The subject has important practical as well as theoretical bearings, for upon the low specific inductive capacity of the insulating material used in telegraph cables and wires depends the completeness of the insulation. It is important theoretically, as furnishing evidence by which to test the truth of the theory that induction is transmitted by strain in a continuous medium, as this requires certain relations between the specific inductive capacities and the refractive indices of transparent non-conductors. The work involved is one of considerable difficulty, and though many able experimenters have occupied themselves with the problem, the results obtained differ widely. Mr. Gordon's chapter is a very full presentation of all that has been done, and is one of the most valuable in the work. Besides this investigation, this division contains a clear and complete description of the electrometer of Sir William Thomson, supplemented with a number of drawings of the entire instrument and its various parts.

In the portion of the work devoted to magnetism, descriptions and illustrations of the best modern instruments for measuring magnetic elements—the Kew unifilar magnetometer and dip-circle, and the dip-circle of Mr. Fox for use at sea—are given, with the full instructions for their use. Electro-kinematics includes the various phenomena of voltaic electricity and electro-magnetism, and contains accounts of a number of important and difficult researches. Among these are the determination of the British Association unit of electrical resistance in absolute measure, Blaserne's experiments upon the extra currents, and the investigations of Faraday, Verdet, Weber, and Tyndall upon diamagnetism. A very full and complete description is given of the researches upon the discharge of the electric spark in different gases and at various pressures, and especially of De la Rue and Müller and Spottiswood upon the character of the discharge in high vacua, and the remarkable investigations of Mr. Crookes upon radiant matter. The experimental inquiries of Ayrton and Perry on the difference of potential produced by the contact of dissimilar metals, in elucidation of the question of the origin of electro-motive force in a battery, are also very fully considered.

The division of electro-optics is concerned with a class of important phenomena touching the relation of electricity and light. A full account is given of the discovery by Faraday of the rotation of a beam of polarized light in a magnetic field; and the further researches of Verdet, Becquerel, Kündt, Röntgen, Dr. Kerr, and the author. Mr. Gordon closes his work with a brief sketch of the electro-magnetic theory of light of the late Professor Maxwell, which he states to be that "electro-magnetic induction is propagated through space by strains or vibrations of the same ether which conveys the light-vibrations, or, in other words, 'light itself is an electro-magnetic disturbance.'" The evidence in favor of the theory is found in the similarity in the mode of propagation of both influences, the approximate equality of their velocity of transmission through space, and in the character of conductors. Light-vibrations are at right angles to the line of propagation, and this condition is shown mathematically to hold in the case of induction. The theory requires good conductors to be opaque, which as a fact they are. The most important evidence that both light and induction ore transmitted by the same ether is, however, found in the fact determined experimentally that they both have sensibly the same velocity in air and a vacuum. The theory might well bear a fuller exposition than Mr. Gordon has made, and many readers will regret that he has not set it forth at greater length.

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