CRYSTALLIZATION EXPERIMENTS 273 



if the first trial fails to yield long hair-like crystals, repeat, changing the relative 

 quantities of the two salts. 



Crystallization of Molten Compounds on Freezing. 



6. Place a large fragment of Thymol (m.p. 50 C.) at the corner of a slide, lay 

 a clean cover-glass upon the fragment and heat over the " micro " flame until the 

 material just melts; remove from the source of heat at once and lay upon the glass 

 plate upon the table. Press down very gently the cover-glass at its center with a 

 glass rod and hold it in position until freezing begins at the periphery of the cover- 

 glass. Place the preparation on the stage of the microscope and watch the crystals 

 grow during freezing. Note well (a) that the growing crystals at their free ends 

 show well developed faces and angles, (6) that the crystals do not penetrate one 

 another, (c) that air is entrained, carried along and as the melt freezes the crystals 

 contain marked air " holes " (inclusions), (d) that as the mass cools and contrac- 

 tion takes place, cleavage planes appear and the crystals become ruptured. 



Remelt the preparation which has just frozen and proceed as before. Repeat 

 several times so as to obtain a thorough conception of the way in which the prepa- 

 ration behaves on freezing. Examine the preparation between crossed nicols 

 during the process of growth and after freezing has ceased. Note the orientation 

 of the crystals and trace out the crystal boundaries. These aggregates correspond 

 to what are called " crystal grains " in metals and alloys. See if you can change 

 the " grain size " by varying the rate of cooling the preparation. These and the 

 following experiments carefully performed will greatly aid the student in a better 

 understanding of the phenomena of freezing in alloys and will enable him to better 

 interpret the microscopic appearance obtained on etching a polished specimen. 



7. Place a fragment of urea (m.p. 132 C.) at the corner of a slide, lay upon it 

 a cover-glass. Melt over the micro flame. Hold down the cover-glass during 

 freezing and when cool study under -the microscope. 



8* Melt and study orthonitrophenol (m.p. 45 C.). 



9. Melt and study sulphonal (m.p. 127 C.). 



10. Melt and study the frozen mass of (a) cobalt nitrate (m.p. 59 C.) (6) 

 nickel nitrate (m.p. 57 C.) (c) Place a fragment of a and a fragment of b several 

 millimeters apart. Cover with a cover-glass and melt carefully the fused drops 

 of two salts should just run together. Note that at the line of juncture the freez- 

 ing is slower than in the pure materials. 



11. Prepare preparations of Naphthalene (f.p. 80 C.) and Phthalic anhydride 

 (f.p. 130.8 C.). Note that where the drops have flowed together, freezing is long 

 delayed (eutectic at 64.9 C., Naphthalene 71 per cent, Phthalic anhydrid 29 per 

 cent). 1 If the phenomena of the eutectic is not seen, repeat the experiment, 

 using different proportions of the two components. Note the differences in the 

 crystals which separate at different points. 



12. Place a small quantity of monochloracetic acid upon a slide, lay a clean 

 cover-glass upon the crystals, heat gently until they melt, being careful to have 

 every particle of the preparation completely melted. Cool rapidly by laying the 

 preparation upon a cold metal surface. After the compound freezes examine under 

 the microscope using crossed nicols. Then with a clean glass rod scratch the mass 

 where it extrudes beyond the circumference of the cover-glass. Note that the mass 



1 Monroe: J. Ind. Eng. Chem. 41 (1919) 1119. 



