xxvii.] GENERALISATION. 609 



Compounds of lead have both a high dispersive and 

 a high refractive index, and in the latter respect thev 

 proved jery 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 

 boracic acid, now known as heavy glass, which possessed 

 an intensely high refracting power. Many years after- 

 wards in attempting to discover the action of magnetism 

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

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

 piece of the heavy glass. The peculiar refractive 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 physical science there is some 

 substance of powers pre-eminent for the special purpose to 

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

 diathermancy or transparency to the least refrangible rays 

 )1 the spectrum. Quartz is equally valuable for its trans- 

 parency as regards the ultra-violet or most refrangible rays 

 Diamond is the most highly refracting substance which is 

 at the same time transparent ; were it more abundant and 

 easily worked it would be of great optical importance 

 Unnaoar is distinguished by possessing a power of rotatino- 

 the plane of polarisation of light, from 15 to 1 7 times as 

 much as quartz. In electric experiments copper is em- 

 ployed fonts high conducting powers and exceedingly low 

 magnetic properties ; iron is of course indispensable for its 

 enormous magnetic powers; while bismuth holds a like 

 I place as regards its diamagnetic powers, and was of much 

 importance in Tyndall's decisive researches upon the polar 

 character of the diamagnetic force. 1 In regard to 

 magne-crystallic action the mineral cyanite is Thinly 

 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 light-giving powers, which 

 are so extraordinary that probably one half of the whole 

 number of stars in the heavens have a yellow tin^e in 

 consequence. 



1 Philosophical Transactions (1856) vol. cxlvi. p. 246. 



E K 



