422 



THE POPULAR SCIENCE MONTHLY. 



a popular work, the matter is presented in a 

 manner to be of interest to others than pro- 

 fessed students of the science, and should be 

 read by those who desire to know something 

 of the methods and appliances of experi- 

 mental research in this branch of knowledge. 



The subject is considered under tle four 

 general divisions of electro-statics, magnet- 

 ism, 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 in- 

 ductive capacity of different insulating sub- 

 stances. The subject has important practi- 

 cal 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 trans- 

 mitted by strain in a continuous medium, as 

 this requires certain relations between the 

 specific inductive capacities and the refrac- 

 tive indices of transparent non-conductors. 

 The work involved is one of considerable 

 diflSculty, and though many able experiment- 

 ers have occupied themselves with the prob- 

 lem, 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 in- 

 vestigation, this division contains a clear and 

 complete description of the electrometer of 

 Sir William Thomson, supplemented with 

 a number of drawings of the entire instru- 

 ment 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 mag- 

 netometer and dip-circle, and the dip-circle 

 of Mr. Fox for use at sea are given, with the 

 full instructions for their use. Electro-ki- 

 nematics includes the various phenomena of 

 voltaic electricity and electro-magnetism, 

 and contains accounts of a number of im- 

 portant 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 investiga- 

 tions of Faraday, Verdet, Weber, and Tyn- 

 dall upon diamagnetism. A very full and 



complete description is given of the re- 

 searches upon the discharge of the electric 

 spark in different gases and at various pres- 

 sures, and especially of De la Rue and Miil- 

 ler and Spottiswood upon the character of 

 the discharge in high vacua, and the remark- 

 able investigations of Mr. Crookes upon ra- 

 diant matter. The experimental inquiries 

 of Ayrton and Perry on the difference of 

 potential produced by the contact of dissimi- 

 lar metals, in elucidation of the question of 

 the origin of electro-motive force in a bat- 

 tery, are also very fully considered. 



The division of electro-optics is con- 

 cerned 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 polar- 

 ized light in a magnetic field ; and the fur- 

 ther researches of Verdet, Becquerel, Kiindt, 

 Rontgen, Dr. Kerr, and the author. Mr. Gor- 

 don 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 prop- 

 agated through space by strains or vibra- 

 tions 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 con- 

 ductors 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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