Natural Dyes 217 



derived from natural sources; but as an elastin stain, and in the 

 Mollier and Kornhauser quadruple stains, the synthetic orcein 

 seems to be preferable. 



For the technic of the MoIIIer quadruple stain, see Staining Procedures, p. IB3-7. 



For the technic of the Kornhauser quadruple stain, see Id., p. IB3-8. 



For the technic of the Tanzer-Unna orcein stain, see Id. p. IB3-I6, 



For Fraenkel's method for elastic tissue, see Staining Procedures p, IB3-I7. 



U35 LITMUS 



The exact composition of litmus is likewise unknown. It is ob- 

 tained from the same lichens as orcein, treating them with lime 

 and potash or soda, in addition to air and ammonia. Its primary 

 colored principle is known as azolitmin; but there must be other 

 colored compounds in litmus, because the color reactions of azo- 

 litmin are not exactly the same as those of litmus. Azolitmin 

 may or may not be a single chemical compound ; and in any case, its 

 chemical formula is unknown. It can be synthesized from 

 orcinol by methods very similar to those used in preparing orcein, 

 and the synthetic product is very much like the azolitmin obtained 

 from the lichens, if not identical with it. The color reactions of 

 either litmus or of azolitmin are very similar to those of the in- 

 dophenols (see p. 102); but it is difficult to figure how an indo- 

 phenol could be derived from orcinol by the process employed. 



Litmus is a feeble dye and is never used as an histological stain. 

 Its classic use is for indicator purposes; at one time it was the best 

 acid-base indicator known. Since the importance of learning 

 actual pH of solutions came to be realized, however, litmus has 

 proved too inaccurate an indicator, and has largely been replaced 

 for chemical use by synthetic dyes (notably the sulfonphthaleins) 

 which change color through a pH-range near the neutral point 

 and which are not subject to loss of color on reduction. 



This loss of color by reduction, which is an inconvenience to the 

 chemist who wishes to use litmus as a pH indicator, happens to be 

 of advantage to the bacteriologist. Litmus has been employed 

 for over 50 years as an indicator in bacteriological media; and in 

 some of these media (especially milk) it serves to indicate changes 

 in pH and in 0-R potential simultaneously. Although chemi- 

 cally speaking, it is a decidedly inaccurate indicator for either 

 purpose, it has served the bacteriologist to bring out differences be- 

 tween species in a way that no other indicator alone has proved 

 able to do. Azolitmin, either the synthetic product or that de- 

 rived from the lichens, does not serve the purpose, chiefly because 

 it does not have exactly the same sensitive range in relation to the 

 pH-scale as does litmus. 



It is of interest to add that chemists have been telling bacteriol- 

 ogists for years that they should discard litmus in favor of more ac- 

 curate indicators, using one for pH, another for 0-R potential. 



