312 ELEMENTARY CHEMICAL MICROSCOPY 



ponent by one of the methods described under Distillation 

 (Chapter XII), the vapor being condensed and fixed in a drop 

 of water (or other solvent) containing some reagent which will 

 tend to " fix " the volatile compound: for example NaOH or 

 KOH for HCN, H 2 S, HCOOH, CH 3 COOH, etc., or HC1 for NH 3 . 

 Only a trace of alkali or of acid is added to the tiny drop of 

 water placed upon an object slide. The slide is then inverted 

 over the watch glass or the crucible which contains the sub- 

 stance to be tested plus the reagent required to liberate the 

 volatile constituent. Gentle warming will accomplish its expul- 

 sion. 



EXPERIMENTS. 



a. Evaporate a drop of Na 2 CO 3 solution. Cover with gelatin, test with HC1. 



b. Place a b'ttle CaCO 3 on an object slide, cover and test as above. 



c. Test a little zinc dust in like manner. 



d. Test a cyanate in like manner, using H 2 SO4. 



XII. An amorphous precipitate is formed by the reagent and 

 requires special treatment to induce crystallization. 



It has already been pointed out that in microchemical quali- 

 tative analysis an amorphous precipitate is the least desirable 

 form in which a substance may be separated for identification. 

 Nevertheless, it often happens that such precipitates are obtained 

 either accidentally or when it is more expedient to thus remove 

 a substance in order to prevent it from interfering in subsequent 

 testing for other substances. 



In qualitative analysis by means of microscopic methods two 

 classes of amorphous precipitates are met with : (a) Those which 

 require solution in a special solvent from which a crystalline 

 compound eventually separates, and (b) those in which crystal- 

 lization can be induced by inoculation with a trace of the same 

 compound in a crystalline condition. 



Special mention is here made of the treatment of amorphous 

 precipitates because in a number of instances treatment with hot 

 concentrated sulphuric or hydrochloric acids must be resorted 

 to in order to obtain recognizable compounds. 



