190 CIRCULAR POLARIZATION. SECT. XXII. 



plate of quartz to be ^ of an inch thick, which will give the red 

 tint to the space within the interior ring ; when the analyzing 

 plate is turned in its own plane through an angle of 17, the 

 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 them- 

 selves differ apparently only by a very slight, almost imper- 

 ceptible, 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 devia- 

 tion 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 solution 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 differ- 

 ence was also observed by M. Biot between the action of the 

 particles of the same substances 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 polari- 

 zation to revolve towards 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, alcohol, &c. which 



