306 ELEMENTARY CHEMICAL MICROSCOPY 



of a crucible, preferably a tiny glass crucible as described on 

 page 293, or in a small beaker. Saturate with alcohol and invert 

 over the test drop. Owing to the difference in the vapor tensions, 

 alcohol will be absorbed by the aqueous solution and the crystal- 

 line phase will rapidly separate. Only a very short exposure is 

 necessary. 



When dealing with very thin films or tiny drops where there 

 is a tendency to evaporate to dryness, exposure to alcohol 

 vapors is especially valuable. 



EXPERIMENTS. 



a. Prepare a large drop of a moderately concentrated solution of Pb(N0 3 )2. 

 From this large drop take two small ones. Allow one of them to evaporate spon- 

 taneously. Treat the other with alcohol vapor as described above. Note the 

 difference in time required for the appearance of crystals. 



b. To a dilute solution of a calcium salt add a drop of dilute H 2 S0 4 by Method /, 

 page 299. Sheaves, bundles and isolated acicular crystals of CaS0 4 • 2 H 2 will 

 separate. Prepare a solution of the calcium salt so dilute that no CaS04 appears 

 after standing two or three minutes. Expose to alcohol vapors and note that 

 characteristic crystals are soon visible. 



VII. The reagent is dissolved in alcohol and a drop of the alco- 

 holic solution is employed as in Method I. 



Although we are here dealing with a mode of applying the 

 reagent already discussed, alcoholic solutions need special men- 

 tion because of the care required in their application. The re- 

 marks which follow are equally applicable to any other solvents 

 or reagents of lower boiling point than water or of different sur- 

 face tensions. 



There is always a marked tendency of the alcoholic reagent to 

 spread over the whole object slide, carrying with it the drop of 

 solution to be tested, or breaking the latter up into so many drop- 

 lets as to render reliable observations impossible. Not infre- 

 quently considerable skill is essential to prevent this dissipation 

 of material. 



When an alcoholic reagent must be added to a reagent drop, 

 always have the drop at the corner of the slide, and tip the slide 

 slightly before the alcohol solution is applied to the glass near 

 the drop; as the reagent leaves the rod or pipette increase the 



