COLORING AGENTS. 55 



volumes of the acid to one of the carmine solu- 

 tion). The addition of the acid should cease before 

 a precipitate begins to form. In order to protect 

 this fluid against changes attributed to bacteria by 

 Hoyer, 1 Dr. Mayer places a small crystal of thymol 

 in the containing bottle ; Hoyer uses chloral-hydrate 

 (i per cent, or more) for the same purpose." 



Weigert 2 prepares the reagent by the following 

 process : " Over 2 gm. of carmine are poured 4 gm. 

 common ammonia, and the whole left twenty-four 

 hours in a place protected against evaporation ; 

 200 gm. of a concentrated picric acid solution are 

 then poured in ; the mixture is left twenty-four 

 hours, until all soluble matters are dissolved. Very 

 small quantities of acetic acid are then added, until 

 a slight precipitate comes down even after stirring ; 

 a rather copious precipitate is usually thrown down 

 in the course of the next twenty-four hours; it 

 should be removed by filtration. A picro-carmine 

 which does not stain readily may be improved by 

 the addition of acetic acid." W. T.] 



H^MATOXYLIN. 3 



This substance is the active principle in the ex- 

 tract of logwood, but is not found in great quantity 

 in the tinctura ligni campeschiani. It may be 



1 Hoyer: Beitr. z. histol. Technik. Biol. Centralblatt, 1882, II., pp. 

 17-19. 



2 Arch, pathol. Anat. (Virchow), 1881, Vol. 84, pp. 275-294. Journ. 

 Roy. Micr. Soc., Feb. 1883, Ser. 2, Vol. III., p. 139. 



3 Frey : Mikroskop, p. 91. Pelletan : Le Microscope, p. 209. Ranvier : 



