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, H2S, HCOOH, CH3COOH, etc., or HCl for NH3. 
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 Na2C03 solution. Cover with gelatin, test with HCl. 
b. Place a httle CaCOs on an object slide, cover and test as above. 
c. Test a little zinc dust in Eke manner. 
d. Test a cyanate in like manner, using H2SO4. 
XII. An amorphous precipitate is formed hy 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 {h) 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. 
