430 COLLOIDS IN BIOLOGY AND MEDICINE 



dyed directly only by certain groups of dyes. On this account he 

 is able to obtain almost uniform dyeing. The biologist, on the con- 

 trary, has to stain much more numerous varieties of tissue and it is 

 quite wonderful what different shadings and even what different colors 

 the individual tissue elements of a tissue often show after treatment 

 with a single stain. If we consider, moreover, that each individual 

 tissue element frequently selects its own particular constituent from 

 a mixture of stains, thus forming the brightest pictures, we shall 

 understand why the chemical theory of dyeing is the most convinc- 

 ing to biologists. 



The following is a resume of the results to date of the investiga- 

 tions upon dyeing with basic and with acid dyes. By adsorption the 

 dye is concentrated upon the tissues, with which a fixation may occur 

 as the result of chemical processes. 



Hitherto we have only considered the so-called substantive dyeing, 

 by which the fabric stains directly in the dye solution without any 

 previous preparation (wool, silk). Vegetable fibers, such as cotton, 

 flax, paper, etc., take up very little color from most dye solutions 

 and do not hold it very firmly; they require a mediator to chain 

 the color to them, namely a mordant. This type of dyeing is known 

 as adjective dyeing, a term introduced into industrial dyeing tech- 

 nology by J. BANCROFT. In biological staining the chief mordants 

 are alum and ferric oxid salts. The combination of mordants with 

 dyes (hemotoxylin, hematin and alizarin colors) are called lakes. 

 W. BILTZ has definitely proved that in addition to physical adsorp- 

 tion, chemical combinations occur in adjective dyeing. 



Histologically, staining with dye mixtures is much employed. P. 

 EHRLICH found that if aqueous solutions of an acid dye, e.g., acid 

 fuchsin or orange G, were mixed with a basic one, e.g., methylene 

 blue or methylene green, so that one remained in excess, then no 

 precipitate was formed. From colloidal solutions of dye mixtures 

 certain tissue elements remove the basic and others the acid dye. 

 It is thus possible to obtain with one solution double stains, or even 

 triple stains (triacid). (See above.) 



According to the investigations of O. TEAGUE and B. H. BuxTON,* 2 

 acid and basic dyes precipitate most completely if they are mixed 

 in equimolecular proportions. An excess of one dye interferes with 

 precipitation, i.e., it acts as a protective colloid, and, in fact, the in- 

 terference zones are wider, the more colloidal the dye. Especially 

 important for the histologist is the fact that highly colloidal dye 

 mixtures are bound more firmly together than those that are slightly 

 colloidal. 



We must consider very critically, microchemical reactions and 



