THE METHODS OF MICROCHEMICAL QUALITATIVE ANALYSIS 309 
Applications of this Method. 
Testing for acidity or alkalinity. 
Litmus-silk 
Congo-red-silk 
Testing for boric acid, borates.Turmeric-viscose-silk 
Group reagent for the heavy metals.Wool-zinc-sulphide 
Test for gold... .Adsorption upon silk, reduction with stannous 
chloride 
For the methods for preparing the fiber,^ see Appendix. 
EXPERIMENTS. 
a. Test a very dilute drop of an acidulated solution with blue litmus-silk. 
b. Test a dilute drop of alkaline solution with red litmus-silk. 
c. Place a drop of a dilute solution of borax upon an object slide, acidulate 
with dilute HCl. Dip into the drop from a wax cone a fiber of turmeric-wool. 
Allow to evaporate spontaneously to dryness. Examine the fiber under the 
microscope. It should have a brownish color. Lay the fiber upon a slide and 
moisten with a lo to 15 per cent solution of NaOH. If borates were present the 
fiber turns a bluish or lavender color. 
d. Into a tiny drop of a solution containing Au, lay a fiber of purified raw silk, 
warm gently until evaporated to dryness; carefully avoid too high a temperature. 
The fiber turns yellow or red. Treat with a dilute solution of SnCb containing a 
little tannic acid. A purple color results, due to the precipitation of metallic gold. 
The beautiful red color of the silk fiber before the reducing agent is added is due to 
colloidal gold; the agglutination of the colloidal particles by the SnCb gives rise 
to larger particles which appear purple. 
IX. The delicacy of the test is increased by taking advantage of 
adsorption phenomena, or the test itself depends upon the adsorptive 
properties of a compound. 
Although reactions of this type are those most frequently 
employed in the differentiation of structures, tissues, cells and 
cell contents in biology, histology and pathology, through the 
use of differentiating stains or dyes, their applications in the 
chemical laboratory to the common problems of qualitative 
analysis are limited. 
The basis for selecting a reaction involving adsorption phe¬ 
nomena or solid solution is that the resulting reaction shall con¬ 
fer upon a practically colorless body a color of sufficient intensity 
to render it more easily discernible. Whenever, therefore, stain- 
1 Chamot and Cole: J. Ind. Eng. Ch. IX (1917). 969; X (1918). <<8, 
