THE METHODS OF MICROCHEMICAL QUALITATIVE ANALYSIS 307 
inclination of the slide at once so as to cause the reagent to flow 
toward the material to be tested. Counteract any tendency of 
the reagent to creep up by immediately increasing the inclina¬ 
tion to an almost vertical position. 
Often the preparation cannot be laid flat upon the stage be¬ 
cause of the instant spreading of the alcoholic solution. In such 
an event, the corner of the object slide holding the liquid is in¬ 
serted in the stage opening and may be held in place by another 
slide placed upon the stage, carrying a piece of “plasticine” 
against which the inclined slide is pressed. The preparation can 
then be examined with a low power, focusing each different area 
as it is brought into the field by means of the stage centering 
screws. 
Because of the difficulties involved in the study of inclined 
preparations it is always better to first evaporate to dryness the 
drop of material to be tested so as to obtain a broad thin film 
(see Method IV) and use a reagent solution made with as dilute 
alcohol as will yield the proper conditions required in the test. 
EXPERIMENTS. 
a. Obtain a thin film of KCl at the corner of an object slide. Place near by 
a drop of an alcoholic solution of freshly prepared sodium bismuth thiosulphate.* 
Tip the slide slightly and draw the reagent across the dry film. 
Yellow monoclinic ^ crystals of potassium bismuth thiosulphate separate. The 
salt is believed to have the formula 
3 (K2S2O3) • Bi2(S203)3 • 2 H2O. 
It is readily soluble in water, almost insoluble in alcohol. 
* The reagent is prepared as follows: Place in a small watch glass (25 mm.) a 
small drop of dilute hydrochloric acid; add repeatedly minute amounts of basic 
bismuth nitrate, warming gently from time to time and stirring thoroughly, until 
a trace of the basic nitrate remains undissolved; now add a bare trace of hydro¬ 
chloric acid; just sufficient to dissolve the little residue of bismuth salt, but no more; 
then add to the preparation a tiny drop of water. A permanent precipitate of 
bismuthyl chloride should result. If the first drop of water does not produce 
a permanent precipitate, another drop must be added. To this latter turbid solu¬ 
tion a saturated solution of sodium thiosulphate is carefully added, with constant 
stirring, a tiny drop at a time, as long as any of the precipitate remains undis¬ 
solved. An excess of sodium thiosulphate is to be avoided. A perfectly clear, 
faintly yellowish solution should result. To this clear liquid add alcohol (95 per 
cent) drop-wise, until a permanent turbidity results, which is in turn cleared up 
by the addition of a single drop of water. 
* Hiiysse, Zeit. anal. Chem., 39 (1900), 9. 
