764 A TEXT-BOOK OF BOTANY. 



benzol) is placed on a slide and the specimen mounted. When 

 nearly dry, scrape oft" the excess of balsam, clean the slide and 

 cover-glass with chloroform or benzol, and ring with cement. 



THE MICRO-POLARISCOPE is a useful accessory in conjunction 

 with the microscope. It is employed in the study of technical 

 products, and is chiefly applicable in the examination of crystals, 

 starch grains and cell- walls. A number of substances, owing to- 

 certain peculiarities of structure, are double-refracting or ANISO- 

 TROPIC, i.e., they polarize light. If the double refraction is strong 

 enough these substances show a play of colors. Of these may 

 be mentioned the raphides and the rosette aggregates of calcium 

 oxalate, cane sugar, citric acid, benzoic acid, caffeine, salicin, aloin, 

 phloroglucin, and the salts of berberine, strychnine, and atropine. 

 The acicular crystals which separate in chloral preparations of 

 gambir also show a play of colors. Among the substances which 

 are anisotropic but give no chromatic effects are starch grains, 

 inulin, mannit, the rhombohedra in catechu and the various types 

 of cell-walls. All substances which form crystals belonging to 

 the isometric system are ISOTROPIC or single-refracting, i.e., do 

 not polarize light, as sodium chloride, the octahedra in gambir, 

 potassium iodide and chloral. 



When glass, which is an isotropic compound, is heated and 

 suddenly cooled it is changed into an anisotropic body. Micro- 

 scopic glass beads formed by quickly cooling very thin pieces of 

 glass show polarization effects similar to those of wheat starch 

 grains. This has led to the supposition that the polarization 

 effects produced by starch grains are due to tension rather than 

 to a crystalline structure. But this point cannot be definitely 

 settled until it has been determined whether any of the substances 

 composing the layers of the starch grains are capable of crystal- 

 lization. 



THE SPECTROSCOPE ix MICROSCOPIC ANALYTICAL WORK. To 

 a limited extent at the present time, and yet very effectively by 

 thos.e who are competent to employ it, the Spectroscope is being 

 employed in the examination of organic coloring substances. This 

 method has the advantage that accurate results can be obtained with 

 small quantities of material. With the proper instruments and 

 with practice one may attain a skill equal to that attained in 



