CRYSTALS UNDER THE MICROSCOPE 183 



or imperfect forms are, however, very common in the case of many compounds and 

 especially where several substances are pfesent in solution. 



The presence of colloids in the solution will in most cases either wholly inhibit 

 crystallization or will so greatly modify the form of the crystals separating as to 

 prevent their being identified as the substance which has crystallized out. In the 

 following experiments the effect of colloids will be seen. 



EXPERIMENTS DEALING WITH ABNORMAL CRYSTALLIZATIONS. 



j. Dissolve two or three fragments of sodium chloride in water, concentrate 

 and examine the crystals separating. Add a drop of a concentrated solution of 

 gum arabic, warm gently until all the salt has dissolved, concentrate to crystalli- 

 zation at as low a heat as possible and note well the change in the character of the 

 crystals separating. Instead of cubes and rectangular plates skeleton forms and 

 dendritic masses appear. 



2. Try a similar experiment using ammonium chloride. In neither case can 

 well-defined crystals be obtained, but the appearance of the dendritic masses has 

 changed under the influence of the colloid. 



3. Place a tiny drop of water on a slide on a piece of white paper, add ferric 

 chloride until a distinct yellow color is obtained. Now add two or three frag- 

 ments of ammonium acetate, stir gently until dissolved, but do not heat. Crys- 

 tallization will soon set in. The crystals of ammonium chloride now separating 

 will be well-formed cubes or the skeletons of cubes. 



4. Try crystallizing mercuric chloride in the presence of gum arabic. 



5. Place next to a drop of a solution of lead nitrate a solution of potassium 

 iodide. Cause the drops to flow together. Note that lead iodide is formed in 

 shining iridescent plates. To a drop of a solution of lead nitrate add a fragment 

 of gelatin, warm gently to dissolve the gelatin. Place a drop of potassium iodide 

 solution next to the test drop just prepared and cause the iodide solution to flow 

 into the test drop. It will be evident that lead iodide has been formed because of 

 the yellow zone at the point of contact, but no crystals will separate. 



6. To a drop of olive or cottonseed oil on a slide add stearic acid and warm 

 gently until the drop is clear. On cooling, radiating masses of thin plates separate. 

 Examine between crossed nicols. 



7. Place a few fragments of sulphonal upon a slide, lay a cover glass upon the 

 material and heat until the sulphonal melts. A very thin molten film of the com- 

 pound results, which crystallizes on cooling. Examine between crossed nicols. 



8. Dissolve a very minute amount of barium chloride at the corner of a slide; 

 add a fragment of sodium acetate. Dissolve a fragment of oxalic acid in a drop 

 of water close to the drop of barium chloride solution. Cause the oxalic acid to 

 flow into the other drop. In a few seconds large branching aggregates in the form 

 of radiating bundles and sheaves of fibrous needles of barium oxalate will be seen. 



Make a fresh solution drop of barium chloride; add sufficient ferric chloride to 

 give the drop a distinct yellow color. Now add sodium acetate as before; stir 

 until dissolved. The drop should now acquire a reddish tint. Into this test drop 

 cause oxalic acid to flow. Long hair-like crystals (trichites) separate instead of 

 branching aggregates. 



9. Prepare a drop of an almost saturated solution of chromium chloride. Add a 

 little solid mercuric chloride and warm gently until dissolved. Concentrate to 

 crystallization. -Trichiten crystals of the double chloride separate. 



70. Prepare a solution drop of sodium asparaginate. Try to obtain crystals by 

 concentration. Start crystallization by "seeding;" that is, crush in the drop the 

 smallest possible piece of the salt. 



