ELECTROSTATICS AND MAGNETISM. 305 



articles, and the tendency to reversion to older types in the absence of scientific 

 control. 



A good deal of Sir W. Thomson's practical electrical work is not referred 

 to in this volume. It is to be hoped that he will yet find time to give some 

 account of his many admirable telegraphic contrivances in galvanometers, sus- 

 pended coils, and recording instruments, and to complete this collection by his 

 papers on electrolysis, measurement of resistance, electric qualities of metals, 

 thermo-electricity, and electro-magnetism in general*. 



The second division of the book contains the theory of magnetism. 



The first paper, communicated to the Royal Society in 1849 and 1850, is 

 the best introduction to the theory of magnetism that we know of. The 

 discussion of particular distributions of magnetisation is altogether original, and 

 prepares the way for the theory of electro-magnets which follows. This paper 

 on electro-magnets is interesting as having been in manuscript for twenty-three 

 years, during which time a great deal has been done both at home and abroad 

 on the same subject, but without in any degree trenching upon the ground 

 occupied by Thomson in 1847. Though in these papers we find several formidable 

 equations bristling with old English capitals, the reader will do well to observe 

 that the most important results are often obtained without the use of this 

 mathematical apparatus, and are always expressed in plain scientific English. 



As regards the most interesting of all subjects, the history of the develop- 

 ment of scientific ideas we know of few statements so full of meaning as the 

 note at p. 419 relating to Ampere's theory of magnetism, as depending on 

 electric currents, flowing in circuits within the molecules of the magnet ; he 

 goes on to say : " From twenty to five-and-twenty years ago, when the materials 

 of the present compilation were worked out, I had no belief in the reality of 

 this theory ; but I did not then know that motion is the very essence of what 

 has been hitherto called matter. At the 1847 meeting of the British Association 

 in Oxford, I learned from Joule the dynamical theory of heat, and was forced 

 to abandon at once many, and gradually from year to year all other, statical 

 preconceptions regarding the ultimate causes of apparently statical phenomena." 



After a short, but sufficient, proof that the magnetic rotation of the plane 

 of polarised light discovered by Faraday implies an actual rotatory motion of 

 something, and that this motion is part of the phenomenon of magnetism, he 



* [See Mathematical and Physical Papers, by Sir W. Thomson, Vol. i., Cambridge University 

 Pi-ess, 1832.] 



VOL. H. 39 



