162 Dr. Herapath on the Optical Properties of 



have a brilliant emerald-green colour, with almost a metallic 

 lustre; they appear like portions of the elytra of cantharides, 

 and are also very similar to murexide in appearance. When 

 examined by transmitted light, they scarcely possess any colour, 

 there is only a slightly olive-green tinge; but if two crystals cross- 

 ing at right angles be examined, the spot where they intersect ap- 

 pears as black as midnight, even if the crystals are not ji^dth of 

 an inch in thickness. (See Plate IV. fig. 1.) If the light used 

 in this experiment be in the slightest degree polarized, as by re- 

 flexion from a cloud, or by the blue sky, or from the glass sur- 

 face of the mirror of the microscope placed at the polarizing 

 angle, 56° 45', these little prisms immediately assume comple- 

 mentary colours. One appears green and the other pink ; and 

 the part at which they cross is a chocolate or deep chestnut-brown 

 instead of black. 



Their optical properties will be more minutely examined here- 

 after. 



Their chemical characters are the following : — 



They are immediately redissolved upon heating the acid liquid 

 to 180°, and recrystallize on cooling ; those formed in the sul- 

 phuric acid solution, if exposed to the air on a narrow slip of 

 glass, upon the concentration of the mother-liquid by evapora- 

 tion, will slowly disintegrate by dissection, and at length dissolve 

 completely. They are also altered by diluting the solution with 

 distilled water, appearing to become disintegrated. The only 

 mode of preparing these crystals as microscopic objects, is cau- 

 tiously to neutralize the excess of acid of the mother-liquid by 

 the addition of liquid ammonia, but to take care that it be 

 added in successive small quantities, short of precipitation of the 

 excess of disulphates of quinine and cinchonine; then depo- 

 siting upon a glass slide with a dropping tube a portion of the 

 fluid charged with these crystals, allowing the crystals to sub- 

 side, gradually removing the fluid by the capillary attraction of 

 bibulous paper, and immediately drying the crystals by a current 

 of cold air. They may then be mounted in Canada balsam in the 

 usual way ; taking care, however, to use no heat in liquefying the 

 balsam ; otherwise the crystals would be immediately destroyed. 



Boiling alcohol readily dissolves these crystals ; a clear orange- 

 yellow solution results; this on cooling deposits crystals in 

 abundance, having the same optical and chemical characters; 

 but they have lost the prismatic form, and now appear as rosettes 

 of minute hexagonal plates, or forms derived from the hexagon 

 by truncation of the angles. Cold alcohol does not dissolve 

 them. 



Sulphuric sether and chloroform appear to have no solvent 

 power over them. 



