MICROSCOPIC TECHNIQUE AND REAGENTS. 767 



CRYSTALLOGRAPHIC METHOD of examination be utilized. This 

 method originated in the examination of thin sections of rocks 

 and it has been possible by this study to identify the numerous 

 rock-forming mineral species. In those species which are mixed 

 crystals, i.e., made up of isomorphous mixtures of two or more 

 components, it has been possible to determine with some accuracy 

 their composition simply by their optical properties, as for exam- 



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FIG. 420. Codeine: x-shaped skeleton crystals from 10 per cent, alcoholic solution. 



pie the feldspars. Furthermore, it has been possible to draw 

 conclusions as to the ultimate composition of rocks and the 

 conditions under which they were formed. 



The value and possibilities of the employment of the crystal- 

 lographic method in biological studies is well exemplified in the 

 recent work of Reichert and Brown, " The Crystallography of 

 the Hemoglobins." By special means individual crystals of the 

 hemoglobins were obtained and by purely crystallographic 

 methods, including a study of the forms and optical properties 

 of such crystals, the hemoglobins of the 200 species of animals 



