96 BIOLOGY AND TECHNIQUE 



The chemical principles which underlie the staining process are still 

 more or less in doubt. 1 Suffice it to say here that most of the dyes in 

 common use by bacteriologists and pathologists are coal-tar derivatives 

 belonging to the aromatic series, all of them containing at least one 

 "benzolring" combined with what Michaelis terms a "chromophore 

 group/' chief among which are the nitro-group (NO 2 ), the nitroso-group 

 (NO), and the azo-group (N = N). Just what the actual process of stain- 

 ing consists in, is a question about which various opinions are held, some 

 believing that the phenomenon is purely chemical, in which a salt is 

 formed by the combination of the dye and the protoplasm of the cells, 

 others that there is no such salt formation, and that the process takes 

 place by purely physical means. To support the latter view it is argued 

 that certain substances like cellulose are stainable without possessing 

 the property of salt formation, and that staining may often be accom- 

 plished without there being a chemical disruption of the dye itself. 

 Michaelis sums up his views by stating that probably both processes 

 actually take place. A dye stuff, as a whole, may enter into and be de- 

 posited upon a tissue or cell by a process which he speaks of as " insorp- 

 tion." In such a case the coloring matter may be subsequently ex- 

 tracted by any chemically indifferent solvent. On the other hand, a dye 

 after being thus deposited upon or within a cell, may become chemically 

 united to the protoplasm by the formation of a salt, and in such a case 

 the color can be removed only by agents which are capable of decoip- 

 posing salts, such as free acids. 



The staining power of any solution may be intensified either by 

 heating while staining, by prolonging the staining process, or by the 

 addition of alkalies, acids, anilin oil, and other substances which will 

 be mentioned in the detailed descriptions of special staining methods. 



One of the most common examples of such an intensified stain 

 is the so-called Loeffler's alkaline methylene-blue. This is made up in 

 the following way: 



Saturated alcoholic solution of methylene-blue, 30 c.c. 

 1 : 10,000 solution potassium hydrate in water, 100 c.c. 



Another solution designed with a similar purpose is the Koch-Ehrlich 

 anilin-water solution. Anilin oil, one part, is shaken up with dis- 

 tilled water, nine parts; after thorough shaking, the mixture is filtered 



1 For comprehensive reviews of the subject, the reader is referred to dissertations 

 such as those of Mann (" Physiol. Hist. Methods and Theory," Oxford, 1902) and 

 of Michaelis (" Einfuhrung in die Farbstoffchemie," etc., Berlin, 1902), 



