630 THE PEINCIPLES OF SCIENCE. [chap. 



The chemist having discovered what he believes to be a 

 new element, will have before him an infinite variety of 

 modes of treating and investigating it. If in any of its 

 qualities the substance displays a resemblance to an ablaline 

 metal, for instance, he will naturally proceed to try whether 

 it possesses other properties of the alkaline metals. Even 

 the simplest phenomenon presents so many points for 

 notice that we have a choice from among many hypo- 

 theses. 



It would be difficult to find a more instructive instance 

 of the way in which the mind is guided by analogy than 

 in the description by Sir John Herschel of the course of 

 thought by which he was led to anticipate in theory one 

 of Faraday's greatest discoveries. Herschel noticed that 

 a screw-like form, technically called helicoidal dissymmetry, 

 was observed in three cases, namely, in electrical helices, 

 plagihedral quartz crystals, and the rotation of the plane 

 of polarisation of light. As he said,^ " I reasoned thus : 

 Here are three phenomena agreeing in a very strange 

 peculiarity. Probably, this peculiarity is a connecting- 

 link, physically speaking, among them. Now, in the case 

 of the crystals and the light, this probability has been 

 turned into certainty by my own experiments. Therefore, 

 induction led me to conchide that a similar connection 

 exists, and must turn up, somehow or other, between the 

 electric current and polarised light, and that the plane of 

 polarisation would be deflected by magneto-electricity." 

 By this course of analogical thought Herschel had actually 

 been led to anticipate Faraday's great discovery of the 

 influence of magnetic strain upon polarised light. He had 

 tried in 1822-25 to discover the influence of electricity on 

 light, by sending a ray of polarised light through a helix, 

 or near a long wire conveying an electric current. Such a 

 course of inquiry, followed up w^ith the persistency of 

 Faraday, and with his experimental resources, would 

 doubtless have effected the discovery. Herschel also 

 suggests that the plagihedral form of quartz crystals must 

 be due to a screw-like strain during crystallisation ; but 

 the notion remains unverified by experiment. 



^ Life of Faraday, by Bence Jones, vol. ii. p. 206. 



