GENERALIZA TION. 261 



already mentioned (vol. ii. p. 235), 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 polarization was 

 discovered. 



In almost every other part of physical science there is 

 some substance of powers pre-eminent for the special pur- 

 pose to which it is put. Rock-salt is invaluable for its 

 extreme diathermancy or transparency to the least re- 

 frangible rays of the spectrum. Quartz is equally valu- 

 able for its transparency, as regards the ultra-violet or 

 most refrangible rays. Diamond is the most highly refrac- 

 ting 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 polarization 

 of light, from 1 5 to 17 times that of quartz. In electric 

 experiments copper is employed for its high conducting 

 powers and exceedingly low magnetic properties ; iron 

 is of course essential for its enormous and almost ano- 

 malous magnetic powers ; whi]e bismuth holds a like place 

 as regards its diamagnetic powers, and was t)f much im- 

 portance in TyndalTs decisive researches upon the polar 

 character of the diamagnetic force. In regard to magne- 

 crystallic action the mineral cyanite is highly remark- 

 able, being so powerfully affected by the earth's magnetism, 

 that when delicately suspended, it will assume, a constant 

 position with regard to the magnetic meridian, and may 

 almost be used like the compass needle. Sodium is dis- 

 tinguished by its unique light-giving powers, which are 

 so extreme that probably one half of the whole number of 

 stars in the heavens have a yellow tinge in consequence. 



It is highly remarkable that water, though the most 

 common of all fluids, is distinguished in almost every 

 m 'Philosophical Transactions,' (1856), vol. cxlvi. p. 246. 



