REAGENTS. 815 



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 

 ANISOTROPIC, 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 crys- 

 tals 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 grain are capable of crys- 

 tallization. 



The MICRO-SPECTROSCOPE is useful in the study of chlorophyll 

 either in the plant cells or in solution. It may also be used in the 

 study of the absorption spectra of other plant color substances. 



