CRYSTALLIZATION EXPERIMENTS 269 



EXPERIMENTS DEALING WITH CRYSTAL FORMS AND OPTICAL 



PROPERTIES. 



The salts given below have been selected as crystalline com- 

 pounds typical of the crystal systems in which they are placed. 

 The student who is a close observer will note not only the general 

 similarity of the crystals of the salts which have been grouped 

 under each crystal system, but also that each salt differs from 

 the others in its system by certain constant and peculiar charac- 

 teristics; so marked is this individualism in the case of certain 

 species that we are often enabled to recognize at once a salt 

 from its appearance when crystallized upon an object slide. 



Make several preparations of each salt studied. Enter into 

 the note book diagrammatic sketches of characteristic, well-devel- 

 oped, normal crystals. In every preparation there will appear 

 innumerable abnormal, malformed, noncharacteristic crystals; 

 the beginner must be on his guard so as not to confuse the typical 

 with the abnormal forms. 



See that the microscope stage is centered and the nicol prisms 

 properly adjusted. Determine and record the behavior of the 

 crystals under crossed nicols. Record the character of their 

 double refraction whether strong or weak. Determine their 

 extinction angles. Make note of any measurable plane angles. 

 Note well the position and intensity of the contour bands. 



To insure uniformity of method in crystallizations performed 

 upon an object slide we may proceed as follows : 



Place a large drop of water at the corner of a small object 

 slide d X ii in.) ; introduce into this drop a fragment of the salt 

 as large as this o. Warm the preparation very gently over the 

 " micro " flame of a burner. Stir until dissolved. Set aside to 

 cool. Usually on cooling a crystalline crust begins to form about 

 the circumference of the drop. If no crust forms warm again 

 inducing evaporation and cool. With a drawn-down glass rod 

 or platinum wire (Fig. 81) held in a vertical position, gently 

 crush some of the crystalline crust and push the crushed par- 

 ticles into the drop, avoiding as much as possible rubbing or 

 scratching the surface of the glass slide. Usually we have to 

 deal with a metastable condition and this " seeding " of the drop 



