494 SPECIAL METHODS OF DISCO YERY. 



vioufcly known. Faraday also investigated the subject, 

 and confirmed Mr. Armstrong's theory of the cause of the 

 electricity. 



Numerous investigations of the peculiar fact, that a 

 piece of iron-stone attracted iron filings, ultimately 

 resulted in the discovery of the entire science of mag- 

 netism. Brugmans, in 1778, and M. Le Bailli, in 1829, 

 had observed the anomalous circumstance, that bismuth 

 and antimony were repelled by a magnet. Becquerel also, 

 in 1827, and Coulomb, previously, had also noticed that a 

 needle of wood was sometimes repelled by the poles of a 

 magnet ; and the former stated that it placed itself 

 parallel to an electric current in a wire. And it was by 

 investigating these statements that Faraday was led to the 

 discovery of diamagnetism, in the year 1846, and to that 

 of the universality of magnetism. The anomalous be- 

 haviour of substances also whilst being weighed in vacuo 

 led Mr. Crookes, by further research, to the discovery of 

 the rotation of bodies by heat. My experiment of the 

 rotation of a metal ball upon a pair of horizontal rails, 

 by means of an electric current, originated in a peculiar 

 phenomenon observed by Mr. Fearn, in his electro-gilding 

 works, viz., that a horizontal brass tube, laid upon two 

 other horizontal brass tubes, on the top of his electro- 

 plating vat, sometimes moved when the current was 

 passed. 



Other new truths will probably yet be found by investi- 

 gating the peculiar circumstance, observed by Willoughby 

 Smith, in 1873, that light alters the electric conduction 

 resistance of selenium ; also the peculiar fact of contrac- 

 tion of iodide of silver by heating, observed by Fizeau ; 

 and a similar one with iron. 1 I have been informed that 



1 See p. 519. 



