114 BIOLOGY AND TECHNIQUE 



(6) The preparation is thoroughly dried by a blotter or between 

 layers of absorbent paper. 



(7) A small drop of Canada balsam is placed upon the film side 

 of the dry cover-slip, which is then inverted upon a slide. The 

 preparation is now ready for microscopical examination. 



The chemical principles which underlie the staining process are 

 still more or less in doubt. 4 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 

 (N0 2 ), the nitroso-group (NO), and the azo-group (N=N). Just 

 what the actual process of staining 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 ihe 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 prop- 

 erty of salt formation, and that staining may often be accomplished 

 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 deposited 

 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 

 extracted 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 decomposing 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. 



4 For comprehensive reviews of the subject, the reader is referred to disserta- 

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

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

 1902). 



