PICRIC-ACID COMBINATIONS. 121 



CHAPTER XIII. 

 COMBINATION STAINS. 



224. 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. 



225. Picric-Acid Combinations. I follow FLEMMING (Zeit. f. 

 wiss. Mik., 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 haematoxylin. It may be 

 used 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 } 163) ; in fact, this practice had 



