Israel in acetic acid solution for staining sections (nuclei staining 

 blue, cytoplasm red); and by Moll, dissolved in weak hydro- 

 chloric acid, for staining sections of embryos. 



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 colored 

 principle is known as azolitmin. 



Litmus scaj-cely needs comment here. It is a feeble dye and is 

 never used as an histological stain. Its classic use is for indicator 

 purposes; but it is now coming to be largely replaced by the various 

 synthetic dyes (especially sulphonphthaleins) which change color 

 thru an hydrogen-ion range near the neutral point. 



Brazilin and Haematoxylin 



The two natural dyes, haematoxylin and brazilin are closely 

 related chemically and upon decomposition yield the two com- 

 pounds, pyrocatechin 



OH 



and pyrogallic acid 



OH 



OH 

 OH 



OH 



Both dves are obtained bv extraction of the bark of certain trees, 

 haematoxylin from logwood and brazilin from brazil wood (red 

 wood). Both trees are legumes and belong to the family Cesal- 

 piniaceae; they are found only in the tropics. Haematoxylin 

 comes from a single species; while brazil wood is a term applied to 

 various different species all yielding brazilin. 



BRAZILIN C. I. NO. I243 



The composition of this substance is supposed to be: 



HO /\ O CH, 



_ C— OH 



CH \ 

 I CH. 



/ \ 



\ / 



I I 

 HO OH 



95 



