METHODS OF INVESTIGATING BACTEKIA. 781 



Among the acid dyes we reckon all those colouring matters 

 in which the staining substance is an acid ; the dye is not 

 necessarily, however, a free acid, nor need it even have an 

 acid reaction, it may be in combination with bases forming 

 salts, such as picrate of ammonia. There are four classes of 

 acid colouring materials, viz. : 1. Fluorescine ; to this group 

 belong fluorescin, pyrosin, eosin (tetrabromfluorescin), and 

 several others. 2. Nitro bodies for example, Martin's yellow 

 (a salt of binitronaphthol), picric acid, aurantia (the ammonia 

 salt of hexanitrodiphenylamine). 3. Sulpho acids for ex- 

 ample, tropaeolin, bordeaux, ponceau ; derivatives of the form 

 of aniline blue which is soluble in spirit ; aniline black, &c. 

 4. Primary acid dyes for example, rosolic acid, alizarine, 

 nitroalizarine, purpurine, coeruleine, possibly ha3matoxyline, 

 &c. 



To the basic dyes belong fuchsine (rosaniline), methyl 

 violet, methyl green (both of them methyl derivatives of 

 rosaniline, the latter being usually contaminated with methyl 

 violet), triphenylrosaniline (impure aniline blue) and its 

 derivatives, cyanine, safranine, magdala ; further, the stains 

 which are more especially employed for colouring bacteria, 

 viz., Bismarck brown, dahlia, and gentian violet. 



The basic colouring matters are not usually sold as free 

 bases, but as salts ; thus fuchsine is sold as hydrochlorate or 

 acetate of rosaniline. 



The basic aniline dyes are almost alone suitable for 

 staining bacteria ; they, however, also stain the nuclei. 

 In order to prevent the occurrence of a diffuse stain of 

 the whole tissue we must treat the stained preparations 

 with solutions which have a greater affinity with the 

 staining material than the tissue, hut less affinity than 

 the micro-organisms (and nuclei of cells) ; such solutions 

 are alcohol and dilute acetic acid. 



Many bacteria only take up few stains well ; and hence 

 in searching for unknown micro-organisms all the various 

 staining materials must he employed with or without the 

 addition of acetic acid, or in faintly alkaline solution ; 

 the spores of bacilli do not take up the colouring matter 

 unless when they are specially treated. In the case of 

 most forms of micrococci all the nuclear stains (carmine, 

 hsematoxyline, basic aniline dyes) are suitable. These 

 organisms can he stained red with the various forms of 

 carmine which stain the nuclei, and also with purpurine, 



