POLARIZED LIGHT. 113 



lized from water, they arrange themselves in tufts, 

 composed of small oblong plates, arranged round 

 a nucleus. Santonine may also be crystallized on 

 a hot slide, when crystals radiating from a centre 

 will be formed. 



Asparagine, an alkaloid obtained from asparagus, 

 crystallizes in diamonds similar to the crystals of 

 Aspartic acid, shown in fig. 3, plate 9. This acic- 

 is obtained from asparagine, but is difficult to pro- 

 cure ; a specimen had therefore better be procured 

 from the dealers in microscopic objects. 



Succinic acid is obtained by the distillation of 

 amber. 



The preparation of slides of paraffine, stearine, 

 margarine, and wax offer no difficulties to the 

 beginner ; all that is necessary is to place a small 

 piece of the material on a warm slide ; then place 

 a thin cover over it, heat the slide until the 

 substance melts, press down the cover, continuing 

 the pressure until the slide is cold; or the slide 

 can be placed at once on the stage of the micro- 

 scope, and the gradual crystallization observed as 

 the slide becomes cold. 



The medium in which a salt is dissolved affects 

 the form and arrangement of the crystals when 

 it is recrystallized. The media affording the best 

 results are gelatine, gum, and albumen. 



The following method will enable the young 

 student to add many beautiful slides to his collec- 

 tion of polariscope objects. Dissolve, with heat, a 

 small piece of gelatine in the test-tube before de- 

 scribed, using a similar quantity of distilled water. 

 In another test-tube make a saturated solution 

 of the salt (sulphate of copper, for example), add a 

 few drops to the gelatine (mix thoroughly, but 

 avoid forming bubbles, stirring it with a glass rod 

 or piece of platinum wire) ; spread a drop on a 

 I 



