314 ELEMENTARY CHEMICAL MICROSCOPY 
EXPERIMENTS. 
a. Add (by Method I) to a drop of BaCL solution a drop of dilute H2SO4, 
evaporate to cause agglutination of the BaS04; add a drop of water, warin 
gently. Decant. Recrystallize the residue from hot concentrated H2SO4 as de¬ 
scribed above. Cool and breathe repeatedly upon the drop. Study the crystals 
as they form. 
b. Repeat, using Pb(N03)2 instead of BaCL. 
c. Precipitate AgCl from a solution of AgNOs. Recrystallize from concen¬ 
trated HCl. 
XIII. The material to be analyzed is exposed to the action of 
vapors or gases, or a reagent is exposed to vapors or gases resulting 
from the action of some compound upon the material to he tested. 
Oxidation of loosely bound sulphur to sulphate can usually be 
accomplished by placing a drop of bromine in a watch glass or 
crucible (use the apparatus, Fig. 154, page 294), inverting the 
drop of a solution of the substance to be tested over the bromine, 
warming gently in the hood and allowing the preparation to stand 
for five or ten minutes in contact with the bromine vapors. 
In many instances, the substance need not even be in solution, 
but can be merely in suspension, provided it is in a finely divided 
condition. No specific directions are necessary other than the 
caution that the inverted drop must never be so large that there 
is danger of its dropping off the object slide. 
Never perform oxidations with bromine save in the hood at a 
distance from all microscopes. 
After exposure to the oxidizing vapors, the slide is removed, 
turned right side up, the excess of bromine expelled in the hood 
by gentle warming and the remaining drop tested for the pres¬ 
ence of sulphates. 
In testing for the presence of a gas, as, for example, hydrocyanic 
acid, the reagent (in this case silver nitrate solution) may be in¬ 
verted over the container in which the gas is liberated, — watch 
glass, crucible or test tube, — or in testing for arsenic through 
the generation of arsine, the gases may be conducted through a 
tiny capillary tube containing a minute crystal of silver nitrate. 
The distilling tube. Fig. 156, page 296, serves as an excellent 
generator for applying this modification of the Gutzeit test for 
arsenic (see Figs. 161 and 162). 
