XX-. II.] GENERALISATION. 609 



Compounds of lead have both a high dispersive and 

 a high refractive index, and in the latter respect they 

 ]irnved very useful to Faraday. Having spent much 

 labour in preparing various kinds of optical glass, Fara- 

 day happened to form a compound of lead, silica, and 

 horacic acid, now known as hcdvy <jlass, which possessed 

 an intensely higli refracting power. Many years after- 

 wards in attempting to discover the ad ion of magnetism 

 upon light he failed to detect any effect, as has been 

 already mentioned, (p. 588), until he happened to test a 

 piece of tlie heavy glass. The peculiar retractive power of 

 this medium caused the magnetic; strain to be apparent, 

 and the rotation of the plane of polarisation was discovered. 



In almost every part of pliysical science there is some 

 substance of powers pre-eminent for the special purpose to 

 which it is put. liock-salt is invaluable for its extreme 

 diathermancy or transparency to the least refrangible rays 

 of the spectrum. Quartz is equally valuable for its trans- 

 l)arency, as regards the ultra-violet or most refrangible rays. 

 Diamond is the most highlv refracting substance which is 

 at the same time transparent ; were it more abundant and 

 easily worked it would be of great optical importance. 

 Cinnabar is distinguished by possessing a power of rotating 

 the plane of polarisation of light, from 15 to 17 times as 

 much as quartz. In electric experiments copper is em- 

 ployed for its high conducting powers and exceedingly low 

 magnetic properties ; iron is of course indispensable for its 

 enormous magnetic powers; while bismuth holds a like 

 place as regards its diamagnetic powers, and was of much 

 importance in Tyndall's decisive researches upon the polar 

 character of the diamagnetic force.^ In regard to 

 magne-crystallic action the mineral cyanite is highly 

 remarkable, being so powerfully affected by the earth's 

 magnetism, that, when delicately suspended, it assumes a 

 constant position with regard to the magnetic meridian, 

 and may almost be used like the compass needle. Sodium 

 is distinguished by its unique li^ht-giving powers, ^vhich 

 are .^o extraordinaiy that probably one half of the whole 

 number of stars in the heavens have a yellow tinge iu 

 consequence. 



' ndlosophicul Traiisaclioiis (1856) vol. c.xivi. p. 246. 



K K 



