ROTATORY POLARIZATION. 245 



The rotatory liquids do not lose their peculiar power 

 (except in degree) by dilution with other liquids not pos- 

 sessing the property ; and they retain it even in the state 

 of vapour. From these and other facts, M. Biot concluded 

 that this property, in liquids, is inherent in their ultimate 

 particles. In this respect the rotatory liquids are essentially 

 distinguished from rock-crystal, which is found to lose the 

 property when it loses its crystalline arrangement. Thus 

 Sir John Herschel observed, that quartz held in solution by 

 potash (liquor of flints) did not possess the rotatory power ; 

 and the same thing has been remarked by Sir David Brewster 

 with respect to fused quartz. 



(249) When two or more liquids possessing this property 

 are mixed together, the rotation produced by the mixture is 

 always the sum, or the difference, of the rotations produced 

 by the ingredients (in thicknesses proportional to the volumes 

 in which they enter the mixture), according as the liquids are 

 of the same, or of contrary denominations. The same law 

 holds good in many cases in which the liquids are chemically 

 united. 



M. Biot has made an important application of this prin- 

 ciple to the analysis of compounds containing a substance 

 possessing the rotatory power combined with others which 

 are neutral, the quantity of which in the compound may 

 (by the principle just stated) be determined, by observing 

 the optical effects of the mixture. This application has 

 been found of much industrial value, in the case of the sac- 

 chorine solutions; and a very ingenious apparatus, called 

 the saccharometer, has been devised by M. Soleil for the 

 purpose. This instrument is founded upon the principle that 

 the rotatory solutions follow the same laws as rock-crystal, 

 in their action upon the light of different colours ; so that it 

 is possible to compensate the effect of the solution by a plate 



