CH. VII MICRO-CHEMISTRY 179 



a fine glass rod push the two drops together. Shortly bundles of 

 needle-like crystals of CaSo 4 ;2H 2 will appear. This is characteris- 

 tic of calcium. 



Lead nitrate, strontium or barium chloride treated in the same 

 way will yield fine amorphous precipitates. The lead sulphate will, 

 however, slowly recrystallize in characteristic forms. 



For this examination a 16 mm. objective and low ocular should 

 be employed. No cover glass is used. 



§ 253. Slides for Microchemistry and their Preparation. — 

 These are the regular 1 x 3 in. slides cut in half. The work is done 

 on one corner to avoid breaking when the slide is heated. It is 

 very important to have the slides clean. The slides are prepared 

 by leaving them over night in cleaning mixture (Ch IX), and then 

 rinsing very thoroughly in distilled water. The slides are then left 

 in distilled water until ready for use. They are then wiped with a 

 clean glass-towel or with a double thickness of gauze. During the 

 whole process the end of the slide to be used must not be touched 

 by the fingers. A drop of water placed on the slide should flatten 

 out and flow evenly over the surface. If it heaps up in a round 

 mass the slide is not clean. 



I 254. The Micro-chemist should be familiar with the appearance of the 

 different crystal forms under the microscope. He should be especially familiar 

 with the appearance of crystals of the chlorids, nitrates, and sulfates of Sodium , 

 Potassium, and ammonium; since some of these salts are sure to appear in 

 almost every test drop examined. The following list of substances have been 

 suggested by Dr. Chamot as giving definite and easily obtained results. To 

 obtain good crystals dissolve a fragment of the substance in a small drop of 

 water or other solvent and let it evaporate spontaneously until crystals appear. 

 It is better to make the microscopic examination before the drying is complete. 

 Do not use a cover-glass. If one does not obtain good crystals, "seed" the 

 solution with some of the crust which forms at the edge of the drop by push- 

 ing some of the crust into the middle of the drop. This usually starts the 

 ■crystallization. 



Frequently a chemically pure salt cannot be made to yield satisfactory 

 crystals on the evaporation of its solution, but beautifully formed crystals will 

 result when in the presence of other compounds. A striking example is found 

 in Ammonium chlorid. This salt fails to yield other than dendritic masses 

 when preparations are made from the pure salt, but if formed by metathesis 

 and especially if in the presence of a difficultly crystallizable salt, well formed 

 isometric crystals (cubes) are seen. 



