272 ELEMENTARY CHEMICAL MICROSCOPY 



tals we might expect to obtain abnormal or malformed or imper- 

 fectly developed crystals (or none at all), forms which we should 

 scarcely think of associating with the compounds present. 



In all of the experiments performed above the solid phase has 

 separated from water or from alcohol, but there are other ways 

 in which crystals may be obtained, one of which at least claims 

 our attention : crystallization of a molten mass as it freezes. 

 Since these are just the sort of phenomena which arise in every- 

 day practice it is important that the chemist shall have had 

 experience with typical examples. 



EXPERIMENTS DEALING WITH RAPID AND ABNORMAL CRYSTAL- 

 LIZATION; CRYSTALLIZATION FROM FUSION; CRYSTAL- 

 LIZATION IN THE PRESENCE OF COLLOIDS, ETC. 



Influence of too Rapid Crystallization upon Crystal Forms. 



1. Dissolve a little potassium antimonyl tartrate in water and obtain crystals 

 as described under the first series of experiments performed. 



Repeat but this time heat to boiling, blow on the preparation to hasten evapor- 

 ation, heat again and again, blow, hasten the evaporation as much as you possibly 

 can. Compare the crystals obtained with those obtained by slow crystallization. 



2. In like manner try mercuric chloride, ammonium sulphate and urea. 

 Influence of the Presence of another Compound on Crystals. 



3. Crystallize urea in the presence of sodium chloride. Note well the change in 

 crystal form. This was long believed to be a case of dimorphism, but later investi- 

 gations indicate the formation of a compound between urea and sodium chloride. 



4. Dissolve a minute quantity of barium chloride in a drop of distilled water, 

 add a tiny fragment of sodium acetate, stir until dissolved. Place a second drop 

 of distilled water about i mm. away from the first. In this second drop dissolve 

 a fragment of oxalic acid. Cause the drop of oxalic acid to flow into the drop of 

 barium chloride. In a few seconds crystals of barium oxalate separate in the 

 form of branching aggregates, radiating bundles and sheaves of fibrous needles. 



Start a new preparation, but after all the sodium acetate has dissolved add suffi- 

 cient ferric chloride to impart to the drop a distinct reddish color. Lay the prepa- 

 ration on a piece of white paper in order that the reddish tint may be distinctly 

 seen. Now cause the oxalic acid to flow in exactly as before. The crystals of 

 barium oxalate will take the form of long curving hairs or bundles and tufts of hair- 

 like bodies (trichiten crystals). Watch the preparation carefully and note that 

 the longer of these hairs curve, bend and sway as they grow closely simulating life. 



5. Prepare a drop of a saturated solution of chromium chloride. Add to the 

 warm drop a fragment or two of mercuric chloride. Warm the preparation gently 

 and set aside to cool. If the proper concentration has been obtained trichiten 

 crystals of a double chloride of chromium and mercury should separate. It is dif- 

 ficult to obtain just the right conditions to lead to the formation of long trichites; 



