416 ELEMENTARY CHEMICAL MICROSCOPY 



it is merely necessary in most cases to reverse the test for the 

 bases to enable us to properly identify the acids. 



The behavior of the crystals, obtained in a test, toward polar- 

 ized light will be found to be of great value in identifying the 

 salts present in a mixture. The student should have acquired 

 therefore, early in the course, the habit of examining his prep- 

 arations between crossed nicols. Proceeding in this manner in 

 connection with the qualitative tests we can usually determine 

 the true nature of the salts present. 



In testing for the acids it is essential that the student shall 

 always. examine the preparations before they evaporate to dry- 

 ness and that he shall carefully observe the various precautions 

 which have been given in the discussion of the various tests for 

 the bases. 



When dealing with an unknown substance first spread out 

 a little of the dry material upon a slide and examine it with 

 a low power. If the material is not homogeneous, endeavor 

 to pick out particles of its different components, using a plat- 

 inum wire or glass rod. Then work upon each component 

 separately. 



Try the solubility in water, acids, etc. 



Test the reaction toward litmus-silk (Method F/77, page 308) 

 or other indicator. 



If the material is crystallizable, make observations as to its 

 probable crystal system. Test the crystals between crossed 

 nicols. 



Finally make rough estimations of the refractive indices by 

 the immersion method or make melting-point determinations, or 

 both, if possible. 



For convenience in microchemically testing for the acids we 

 may make use of the following slight modification of the Bunsen- 

 Treadwell classification of the acids, based upon the behavior 

 of their salts toward silver nitrate, and toward barium chloride, 

 in neutral and in nitric acid solutions. 



In case a free acid is to be dealt with it is best to add ammo- 

 nium hydroxide in slight excess and drive off the excess, after neu- 

 tralization, by evaporation to dryness. Then proceed as follows : 



