426 COLLOIDS IN BIOLOGY AND MEDICINE 



hand an insoluble compound (textile fiber-dye) and on the other a 

 soluble compound which passes into the bath, e.g., 



wool + rosanilin hydrochloric! = wool rosanilin base + ammonium chlorid. 



The adherents of the adsorption theory insist that the dye salts 

 are frequently strongly hydrolyzed in solution, so that there is, 

 accordingly, only an adsorption of the color base or color acid, but on 

 this account no chemical double decomposition is required in the 

 staining, inasmuch as fibers, as well as dye, are often colloids of 

 opposite charge which mutually precipitate each other. In favor of 

 this view is the fact that a dye solution stains the better, the more 

 colloidal it is. The numerous minute additions employed in micro- 

 scopical stains (methylene blue with a trace of alkali, gentian violet 

 with anilin water, etc.) are usually added for the purpose of making 

 from a true solution one less dispersed. P. G. UNNA, who first 

 recognized this, applied the characteristic term "incipient precipita- 

 tion " to the condition of a staining fluid most suitable for staining. 

 Finally, we shall indicate another point made by adherents of the 

 adsorption theory; that in those cases in which hydrolytic cleavage 

 is not demonstrable, we frequently observe, not only in the case of 

 textile fibers but also in adsorption by charcoal and silicates, that 

 with the taking up of the dye, a cleavage of the dye salt occurs 

 (demonstrated for crystal violet, fuchsin, etc.), whereby the cation 

 (dye base) goes to the adsorbent and the anion goes into the solution. 

 (This is chiefly true of basic dyes.) 



Such phenomena were first demonstrated by J. M. VAN BEMMELEN 

 in the adsorption of potassium sulphate by hydrated manganese 

 dioxid; free sulphuric acid is found in the solution while KOH is 

 adsorbed. MASIUS* has shown in the case of the very strongly 

 hydrolyzed anilin salts, that more anilin than acid is adsorbed by 

 charcoal. 



To explain these cleavage processes, especially in dyeing, it must be 

 assumed that the easily adsorbable dye ion displaces a cation K, Na 

 or the like, which is already present and adsorbed, though possessing 

 little capacity for adsorption. 



The weak point in the argument for the chemical theory resides 

 in the fact that we do not know the real constitution of the adsorbent, 

 that is, the fiber (silk, wool, cotton, etc.,), so that we do not know what 

 chemical groups are involved in a dye combination. H. BECHHOLD 

 accordingly strove to solve this question by using as adsorbent a 

 group of substances whose composition is accurately known, namely, 

 naphthalin Ci H 7 OH, naphthylamin Ci H 7 (NH 2 ) and amidonaph- 

 thol Ci H 6 OHNH 2 . The result of this experiment is reproduced on 



