ROTATORY POLARIZATION OF LIQUIDS. 141 



the rotation of tlic analyzer. But these figures will alter their forms if the glass 

 be broken, which is not true of the rings formed in crystal. When the heat 

 has fully peneti-ated the glass, and the interior temperature is uniform, the 

 figures cease to be seen. At this time, if the heated glass be removed from the 

 bath, and alloAved to cool rapidly, a new system of figures will spring up Avilhin 

 it. This is related to the former one, as the rings of a positive crystal are to 

 those of a negative one ; and, therefore, if two similar solids, in one of which 

 the former set of figures is seen, and in the other the latter, be superposed when 

 the intensities are equal, they Avill neutralize each other's effects, and the rings 

 will disappear. This structure may be made permanent in the glass solids wc 

 have been considering by heating them nearly to the point of fusion and then 

 suddenly cooling them. Many common articles of glass are so imperfectly an- 

 nealed as to display the doubly refracting structure in a striking manner. The 

 stoppers of bottles, if cut across the axis and polished, will invariably show it ; 

 so will the stems of wine glasses, the stirring-rods of the chemist's laboratory, 

 and mr.ny, if not all, glass tubes. 



The effects of heat are also remarkable in altering the doubly refracting 

 character of crystals. Mr. Mitscherlich discovered that heat expands crystals 

 uneqnaljy in different directions. Iceland spar is expanded in the direction 

 of its axis, and slightly contracted at right angles to the axis. Its doubly re- 

 fracting power is thus diminished. In sulphate of lime, which is a crystal of 

 two axes inclined to each other 60°, he found that the inclination diminishes 

 with elevation of temperature, until the two axes unite in one; after which, 

 with further increase of heat, they open out in a plane at right angles to the 

 first. Dr. Brewster discovered an example even more remarkable in glauherite. 

 At the freezing point, this crystal has two optic axes for every color of the 

 spectrum, the inclination of the axes of the red being greatest, and that of the 

 violet being least. At ordinary temperatures it has two axes for red and one 

 for violet. When heat is applied, the other axes approach, as in the case just 

 described, and, after successively uniting, successively open out in the transverse 

 plane. 



In comparing the crystals which possess the power of double refraction. ( being 

 by far the greater number of the Avhole, ) there is found to be a certain relation 

 between the optical character of the crystal and the crystallographic structure. 

 All crystals whose primitive form is the cube, the regular octahedron, or the 

 rhomboidal dodecahedron — figiu-cs whose geometrical axes are all equal — are des- 

 titute of the property. All crystals which have one axis greater or less than the 

 others are crystals of one optic axis. All crystals whose geometric axes are all 

 three unequal have two axes of double refraction. 



In the year 1815, Mr. Biot made the remarkable discovery that many liquids 

 possess the power of rotatory polarization — a discovery which was independently 

 made by Mr. Seebeck ; the effect was first observed in oil of turpentine, but has 

 since been found in most essential oils, in solutions of sugar, dextrine, the vege- 

 table alkaloids, camphoric and tartaric acid, and the tartrates. In some of these 

 substances the plane of polarization is turned to the right and in others to the 

 left. Their relative rotatory forces are estimated by a comparison of the amount of 

 angular change in azimuth produced upon a polarized ray in passing through a 

 column of given length ; but as yet there has been no universal agreement upon 

 a standard length. The statements of experimenters, therefore, usually embrace 

 both the angular rotation and the length of the column by which it has been 

 produced, rendering a reduction to a common length necessary before a correct 

 comparison can be instituted. It would perhaps be most convenient to adopt 

 as a standard length, the length of the tube introduced by Mr. Solcil into his 

 saccliarimeter, or instrument for measuring the rotation in solutions of sugar, 

 which is twenty centimetres. With this length the dextro-gyration of the oil 

 of bitter oranges is, for red light, 157°.89, which is the maximum observed in 



