PICRIC ACID COMBINATIONS. 153 



CHAPTER XIII. 



COMBINATION STAINS. 



235. The Classes of Multiple Stains. I distinguish two 

 classes of multiple stains. In the one a pure nuclear stain, 

 taking effect on all the nuclei of all the tissues of a prepara- 

 tion, is combined with a stain taking effect on all the extra- 

 nuclear parts of all the tissues. Borax-carmine followed by 

 indigo-carmine is a typical example of such a combination. 

 In the second class, a stain taking effect on the totality of the 

 elements of any one tissue exclusively is combined with a stain 

 or stains of another colour taking effect on the totality of the 

 elements of the other tissues. 



The first class, aiming at enhancing the usefulness of a pure 

 nuclear stain by improving the definition of extra-nuclear 

 parts, has a legitimate scientific end in view, and is capable 

 of rendering service in research. It will, therefore, here be 

 treated much more fully than the second class, which is com- 

 posed of much less generally useful, and too frequently merely 

 ornamental stains. 



236. Picric Acid Combinations. I follow FLEMMING (Zeit. f. 

 wiss. Mik.y i, 1884, p. 360) in pointing out that picric acid is 

 perhaps the most generally useful of all secondary stains. It 

 gives useful plasma stains with most of the nuclear stains, 

 and particularly with carmine and hsematoxylin. It may be 

 lised with the most delicate of these stains, even the delicate 

 coloration of alum-carmine being in no wise injured by it. 

 The modus operandi is as simple as possible ; it consists merely 

 in adding picric acid to the alcohols employed for dehydrating 

 the objects after staining with a nuclear stain. 



Care must be taken in adding picric acid to alcohol acidu- 

 lated with HC1 (see ante, 169) ; in fact, this practice had 

 better in general be avoided, and the picric acid only added 

 to the pure alcohol used after washing out. 



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