PHYSICS OF NINETEENTH CENT. ELECTRICITY. 589 



is by means of electro-magnets of the largest dimensions, actuated by 

 strong battery currents, that most of the facts have been ascertained. 

 No doubt it was owing to the comparative weakness of the dia-mag- 

 netic forces that the phenomena were so long overlooked. The great 

 importance to science of these discoveries consists in this : that 

 whereas formerly magnetism was known only as an exceptional pro- 

 perty of iron and of two or three other metals, it must now be recog- 

 nized as a common property of all matter. The march of science is 

 ever towards greater generality, hence the significance of the step by 

 which a certain property is raised from being possessed by only par- 

 ticular substances into a universal property differing but in degree and 

 direction from one body to another. 



A theory the counterpart of that of Ampere, which so beautifully 

 embraces all the facts of magnetism, electro-magnetism, and magneto - 

 electricity, has been proposed in explanation of the facts of dia-mag- 

 netism. The currents of the inducing magnet are supposed in the 

 case of dia-magnetic bodies to turn the currents in the contrary direc- 

 tion to themselves ; but no satisfactory explanation has been offered 

 of the cause of this inversion of the direction. Faraday, however, 

 was of opinion that dia-magnetic bodies were destitute of polarity, the 

 law of their action being the simple repulsion of their particles by the 

 poles of a magnet. The question of polarity or non-polarity gave rise 

 to much discussion, and it was not finally set at rest until after an in- 

 vestigation of the subject by Professor Tyndall (page 475), who, by 

 elegant and conclusive experiments, demonstrated in 1856 the exist- 

 ence of both attractive and repulsive forces in dia-magnetic bodies. 



The most notable steps in the progress of the science of electricity 

 and magnetism (for now these are one) during the nineteenth century 

 have now been described. They are these : 



1800. The voltaic pile (or galvanic battery) invented by Volta. 

 1820. Electro-magnetism discovered by CErsted, Arago, and Am- 

 pere, and the mechanical actions between currents studied 

 by the latter. 

 1831. Magneto-electricity and the induction of currents discovered 



by Faraday. 



1845. Faraday investigates dia-magnetism and demonstrates the 

 existence of magnetic properties in all matter. 



It was a favourite idea of Faraday's that the various forces of nature, 

 electricity, magnetism, heat, light, gravity, etc., are transmutable and, 

 in a sense, identical. This theoretical notion was the starting-point 

 of many of his experimental researches, and often when baffled by 

 non-success, he would renew his attempts to discover the connection 

 between the various forms of forces. That he did succeed in breaking 

 down many of the partitions between this and that science, and that 

 his discoveries tend largely to support the idea of unity in nature, is 



