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 (i X li 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. 8i) 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 
