APPENDIX 
447 
Preparation of Fibers Impregnated with Litmus. — A good 
quality of raw silk is boiled in water containing a little soap, 
rinsed thoroughly and placed for two hours at room temperature 
in a sodium hydroxide solution containing lo grams sodium 
hydroxide in loo c.c. of water. The silk is then thoroughly 
washed with distilled water. Dyeing this treated silk ^ in, a 
lo per cent solution of purified litmus, acidified with 3 or 4 drops 
of I : 4 sulphuric acid, produces a fiber of the proper color inten¬ 
sity. In order to dye the silk properly, the acid litmus solution 
containing it is evaporated to a thick syrup, the silk then removed 
and washed in running water, neutralized carefully with very 
dilute sodium hydroxide solution and again washed thoroughly. 
If red and blue varieties of the silk are desired, these neutral 
tinted fibers may be treated with dilute acetic acid for red or 
with dilute sodium hydroxide for blue and then washed thor¬ 
oughly in running water. 
The sensitiveness of the litmus silk depends upon the degree 
of adsorption of the dye, the degree of purification of the raw 
silk and the degree of purification of the litmus. 
If too little dye is adsorbed the color change is not distinct 
enough. If too much dye is adsorbed the fiber becomes less 
sensitive and the color is so deep that it renders the fiber opaque. 
The greater the degree of purification of the litmus the more 
sensitive the dyed fiber, though this factor is not as important 
as the two former ones. 
Preparation of Purified Litmus. — The following procedure 
(essentially Wartha’s ^ method) is suggested for obtaining an 
exceedingly pure litmus. Commercial litmus “ cubes are 
extracted with 95 per cent alcohol until the alcoholic extract 
no longer has a reddish tinge. They are then repeatedly 
extracted with water until the greater part of the coloring 
matter is removed, a current of air being blown through the 
solution to prevent reduction. The filtered solution is carried 
to a thick syrup in an evaporator on the water bath. The mass 
is then evaporated several times with portions of absolute alcohol 
‘ Chamot and Cole; J. Ind. Eng. Chem., IX (1917), 969. 
2 Ber., 9 (1876), 217. 
