588 THE PRINCIPLES OF SCIENCE. [CHAP. 



so long as the results of experiment do not clearly negative 

 the notions entertained. In purely quantitative questions, 

 as we have seen, the absence of apparent effect can seldom 

 be regarded as proving the absence of all effect. Now 

 Faraday was convinced that some mutual relation must 

 exist between magnetism and light. As early as 1822, he 

 attempted to produce an effect upon a ray of polarised light, 

 by passing it through water placed between the poles of a 

 voltaic battery ; but he was obliged to record that not the 

 slightest effect was observable. During many years the 

 subject, we are told, 1 rose again and again to his mind, 

 and no failure could make him relinquish his search after 

 this unknown relation. It was in the year 1845 that he 

 gained the first success; on August 3oth he began to 

 work with common electricity, vainly trying glass, quartz, 

 Iceland spar, &c. Several days of labour gave no result ; 

 yet he did not desist. Heavy glass, a transparent medium 

 of great refractive powers, composed of borate of lead, was 

 now tried, being placed between the poles of a powerful 

 electro-magnet while a ray of polarised light was trans- 

 mitted through it. When the poles of the electro-magnet 

 were arranged in certain positions with regard to the 

 substance under trial, no effects were apparent; but at 

 last Faraday happened fortunately to place a piece of 

 heavy glass so that contrary magnetic poles were on the 

 same side, and now an effect was witnessed. The glass 

 was found to have the power of twisting the plane of 

 polarisation of the ray of light. 



All Faraday's recorded thoughts upon this great experi- 

 ment are replete with curious interest. He attributes his 

 success to the opinion, almost amounting to a conviction, 

 that the various forms, under which the forces of matter 

 are made manifest, have one common origin, and are so 

 directly related and mutually dependent that they are 

 convertible. " This strong persuasion," he says, 2 " extended 

 to the powers of light, and led to many exertions having 

 for their object the discovery of the direct relation of light 

 and electricity. These ineffectual exertions could not 

 remove my strong persuasion, arid I have at last suc- 



1 Bence Jones, Life, of Faraday, vol. i. p. 362. 



2 Ibid. vol. ii. p. 199. 



