38 TEXT-BOOK OF BACTERIOLOGY. 



In tissues especially, it is all but impossible to find and to dis- 

 tinguish them, and therefore we must proceed to the observation 

 of stained micro-organisms. 



III. STAINS AND STAINING. 



By the examination of unstained bacteria in the hanging drop, 

 the peculiarities of form are, for the most part, imperfectly recog- 

 nized, and many distinctive marks altogether unseen; the prepara- 

 tions are not very durable, and therefore of but little use for com- 

 parative investigation. This is very different, however, as soon as 

 the distinguishing power of color is utilized. 



In the first place, color is an invaluable means for distinguishing 

 bacteria with certainty from their non-bacterian surroundings, for 

 a certain class of staining matters and the great mass of the bac- 

 teria stand in special mutual relation to each other, which can be 

 utilized at pleasure. Thus the staining of many micro-organisms 

 was the first thing that betrayed their existence, and the true 

 sources of the most important pathological conditions now recog- 

 nized as bacterial diseases have been discovered only by their aid. 



Scarcely a quarter of a century has elapsed since the first faint 

 attempts were made to imbue animal and vegetable tissues with 

 coloring matters. Yet these beginnings for some time attracted 

 but little attention, and it was not till about the year 1875 that the 

 new process came into pretty general use. Then came the anilin 

 colors and their application for the staining of bacteria. Rapid 

 progress was now made; everj'^body began to stain, the isolated 

 and the double staining were introduced, and at the present day 

 the art of staining has already reached a high state of perfection. 

 The names of Weigert, Koch, and Ehrlich are closely connected 

 with these advances in the technics of investigation. 



The anilin colors, which are of special importance for bacteri- 

 ology, are obtained from a secondary product which arises in the 

 manufacture of the gas which lights our streets — from coal tar. 

 This is a somewhat intricately compounded substance, and the 

 number of dyes obtained from it is considerable. 



Most of them have figured in the service of science for a longer 

 or shorter period, and have been adopted on the recommenJation 

 of an investigator who was partial to them. Yet for ordinary pur- 

 poses only a limited number of them have remained in use. 



In an undissolved state most of them are fine, smooth powders, 

 while some occur in the form of small crystalline scales, with an 

 iridescent gleam. 



