STAINING AND EXAMINATION OF MICRO-ORGANISMS 43 



may actually serve to discriminate between different 

 kinds of micro-organisms themselves. 



Even in the dyeing of the textile fibres there is still a 

 very considerable amount of empiricism necessary, and 

 it is not surprising that this should be even more the 

 case in the dyeing of microscopic bodies ; the art of 

 successfully staining bacteria under varied conditions 

 is therefore one which can alone be acquired by pro- 

 longed practice. In the following pages we have 

 endeavoured to present as concisely as possible the 

 more important methods at present in vogue, arranging 

 them in such a manner as to indicate the relationship 

 subsisting between them, and the particular advantage 

 attaching to each. 



Composition and Preparation of various Stains 



Of the innumerable dyestuffs which are at present 

 known, practically only the so-called basic aniline colours 

 are employed in the staining of bacteria, having been 

 first used for this purpose by Weigert (1875). 



These basic coal-tar dyes, like magenta, methyl violet, 

 Bismarck brown, exhibit a strong affinity for the pro- 

 toplasmic contents of bacterial cells as well as for the 

 nuclei of animal tissues, both of which they stain with 

 great intensity. 



The acid coal-tar colours, like eosine, acid magenta, 

 safranine, picric acid, &c., on the other hand, do not 

 exhibit this special affinity for the nuclei and bacteria, 

 and on being applied, for instance, to a section of animal 

 tissue they stain the latter throughout its entire extent. 

 The natural acid dyestuffs, like hasmatoxylin (logwood) 

 and carmine (cochineal), behave also, on the whole, in 

 much the same way. 



These two classes of dyestuffs, the basic and the 

 acid, are therefore sharply distinguished from each 



