184 CIRCULAR POLARIZATION. SECT. XXII. 



red hue vanishes. If a plate of rock crystal ^ of an 

 inch thick be used, the analyzing plate must revolve 

 through 35 before the red tint vanishes, and so on ; 

 every additional 25th of an inch in thickness requiring 

 an additional rotation of 17^ ; whence it is manifest 

 that the plane of polarization revolves in the direction 

 of a spiral within the rock crystal. It is remarkable 

 that in some crystals of quartz, the plane of polarization 

 revolves from right to left, and in others from left to 

 right, although the crystals themselves differ apparently 

 only by a very slight, almost imperceptible variety in 

 form. In these phenomena, the rotation to the right is 

 accomplished according to the same laws, and with the 

 same energy, as that to the left. But if two plates of 

 quartz be interposed which possess different affections, 

 the second plate undoes, either wholly or partly, the 

 rotatory motion which the first had produced, according 

 as the plates are of equal or unequal thickness. When 

 the plates are of unequal thickness, the deviation is in 

 the direction of the strongest, and exactly the same 

 with that which a third plate would produce equal in 

 thickness to the difference of the two. 



M. Biot has discovered the same properties in a 

 variety of liquids. Oil of turpentine, and an essential 

 oil of laurel, cause the plane of polarization to turn to 

 the left, whereas the syrup of sugar-cane, and a solu- 

 tion of natural camphor by alcohol, turn it to the right. 

 A compensation is effected by the superposition or 

 mixture of two liquids which possess these opposite 

 properties, provided no chemical action takes place. A 

 remarkable difference was also observed by M. Biot 

 between the action of the particles of the same sub- 

 stances when in a liquid or solid state. The syrup of 

 grapes, for example, turns the plane of polarization to 

 the left as long as it remains liquid ; but as soon as it 

 acquires the solid form of sugar, it causes the plane of 

 polarization to revolve toward the right, a property 

 which it retains even when again dissolved. Instances 

 occur also in which these circumstances are reversed. 



A ray of light passing through a liquid possessing the 

 power of circular polarization is not affected by mixing 

 other fluids with the liquid such as water, ether, alco- 



