32 METHODS OF EXAMINATION. 



same colour, yield it much more easily to one reagent than to another 

 In working with unknown bacteria, therefore, it is necessary to experi- 

 ment in turn with the various colours, and then the various alkaline 

 and acid decolorising reagents, and to note the reaction in each case. 

 A certain amount of discretion should also be used in the after treat- 

 ment of stained specimens, for although some of the aniline colours 

 are only slightly soluble in clove oil, others are extremely soluble, 

 though they are, in turn, unacted upon by turpentine, xylol, oil of 

 bergamot, or cedar oil. Even the solvent for the Canada balsam should 

 be a matter for careful consideration. Chloroform is perhaps the worst 

 of all. When aniline stained specimens are to be mounted, its place 

 should be taken by turpentine, benzole, and xylol, or the balsam 

 should be simply warmed and used without any solvent. These 

 matters appear trifling in themselves, but upon a discriminating carry- 

 ing out of them depends the success of the operator in obtaining 

 permanent and reliable preparations. 



Fliigge, 1 on the authority of Ehrlich, classifies the aniline colours 

 into two distinct groups, each of which has very distinct chemical and 

 physiological characteristics, — the acid and the basic dyes. 



In the first group the colouring matter acts as an acid in combining 

 with bases to form salts, although it does not necessarily give an acid 

 reaction, nor is it necessarily in the form of a free acid. Amongst the 

 more important are eosin, picric acid, aniline black, alizarin, purpurin, 

 and perhaps hematoxylin. 



To the second group — the basic dyes — belong by far the greater 

 number of those which are used in staining bacteria. These are 

 fuchsin, rosaniline, methyl violet, methyl green, Magdala red, and 

 especially Bismarck brown, dahlia, and gentian violet. These basic 

 colours are sold as salts, and not as free bases, whilst fuchsin, for ex- 

 ample, may be sold as an acid salt, as chloride or acetate of rosaniline. 

 With these nucleus and germ tinting reagents we can colour both 

 nuclei and bacilli : — 



Red with nucleus tinting carmine solutions, with fuchsin, ma- 

 genta, &c, 



1 " Handbuch der Hygiene," Pt. I., FermcuU und Microparasiten, C. Flugge, 

 Leipzig, 1883, p. 2S7. 



