94 MICROSCOPIC METHODS 



preparations stained with aniline dyes are apt to fade when 

 mounted in it. It is accordingly a great advantage to use the 

 acid-free balsam supplied by Griibler. Paraffin sections can 

 usually be dehydrated and cleared by the mixture of aniline oil 

 and xylol alone. 



Sections stained for bacteria should always be cleared, at 

 least finally, in xylol, for the same reason that xylol balsam is 

 to be used for mounting films, viz. that it dissolves out aniline 

 dyes less readily than such clearing reagents as clove oil, etc. 

 Xylol, however, requires the previous dehydration to have been 

 more complete than clove oil, which will clear a section readily 

 when the dehydration has been only partially effected by, say, 

 methylated spirit. If a little decolorisation of a section is still 

 required before mounting, clove oil may be used to commence 

 the clearing, the process being finished with xylol. With a little 

 experience the progress, not only of these processes but also of 

 staining, can be very accurately judged of by observing the 

 appearances under a low objective. 



THE STAINING OF BACTERIA. 



Staining Principles. To speak generally, the protoplasm of 

 bacteria reacts to ^stains in a manner similar to the nuclear 

 chromatin, though sometimes more and sometimes less actively. 

 The bacterial stains par excellence are the basic aniline dyes. 

 These dyes are more or less complicated compounds derived 

 from the coal-tar product aniline (C (5 H 5 . NH 2 ). Many of them 

 have the constitution of salts. Such compounds are divided 

 into two groups according as the staining action depends on the 

 basic or the acid portion of the molecule. Thus the acetate of 

 rosaniline derives its staining action from the rosaniline. It is 

 therefore called a basic aniline dye. On the other hand, 

 ammonium picrate owes its action to the picric acid part of the 

 molecule. It is therefore termed an acid aniline dye. These 

 two groups have affinities for different parts of the animal cell. 

 The basic stains have a special affinity for the nuclear chromatin, 

 the acid for the protoplasm and various formed elements. Thus 

 it is that the former the basic aniline dyes are especially the 

 bacterial stains. 



The number of basic aniline stains is very large. The following are 

 the most commonly used : 



Violet Stains. Methyl- violet, R-5R (synonyms : Hoffmann's violet, 

 dahlia). 



Gentian-violet (synonyms : benzyl-violet, Pyoktanin). 



