266 ELEMENTARY CHEMICAL MICROSCOPY 



EXPERIMENTS. 



a. Add (by Method /) to a drop of BaCl 2 solution a drop of dilute H 2 SO4, 

 evaporate to cause agglutination of the BaSO 4 ; add a drop of water, warm 

 gently. Decant. Recrystallize the residue from hot concentrated H 2 SO 4 as de- 

 scribed above. Cool and breathe repeatedly upon the drop. Study the crystals 

 as they form. 



b. Repeat, using PbNO 3 instead of BaCl 2 . 



c. Precipitate AgCl from a solution of AgNOs. Recrystallize from concen- 

 trated HC1. 



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 be 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. 131, page 246), 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. 133, page 248, serves as an excellent 

 generator for applying this modification of the Gutzeit test for 

 arsenic (see Figs. 138 and 139). 



