STAINING 131 



by appropriate means, we find that the dye itself has a negative 

 charge. Hence there are repellent forces acting between the dye 

 and the paper. Su}ipose. however, that we have also present the 

 ions into which a neutral salt dissociates. The cations, being 

 positively charged, are deposited on the surface of the paper, 

 decreasing or annulling its negative charge and reducing the free 

 ^ energy. There is now little or no obstacle to the adsorption of 

 the d}'e. 



Turning now to the basic dyes, we find that the paper is the 

 more deeply stained the lower the concentration of salt present. 

 According to Freundlich. it is the colour base that is chiefly 

 adsorbed in this case. As was pointed out above, these dyes are 

 hydrolytically dissociated, so that free base is present. This free 

 base, being insoluble, is in the colloidal state, and, like colloidal 

 bases in general, has a positive charge, due to electrolytic dis- 

 sociation of the surface of the particles. See Hardy in Van 

 Bemmelen Gedenkboek, p. 188. Thus, not only are the coloured 

 ions, in this case the cations, strongly adsorbed by the negative 

 paper, but the free base is also. Since foreign electrolytes diminish 

 the charge on the paper, their effect on staining by basic dyes is 

 naturally of the opposite kind to that described in the case of the 

 acidic dyes. The effect of alcohol is in the same direction as that 

 of electrolytes, since it also decreases the electric charge and, there- 

 fore, the amount of dye adsorbed. Facts of the kind referred to 

 in the preceding statements have given rise to an " Electrical 

 Theory of Dyeing," which may account for a larger number of 

 them than any other single theory is able to do. For further 

 particulars of theories and facts relating to dyeing and staining, 

 the reader is referred to Alfred Fischer's Fixirung, Fdrbung 

 uyid Bail des Protoplasmas, Jena, 1899 ; Pelet-Jolivet's Theorie 

 des Farbeprozesses, Dresden, 1910 ; First Report on Colloid 

 Chemistry, Brit. Ass., 1917 ; Gee and Harrisox, Trans. Faraday 

 Soc, vol. vi, 1910 ; Harrisox, Journ. Soc. Dyers and Colourists, 

 December, 1911 ; Bayliss, Biochem. Journ., vol. i, 1906, p. 175. 



The above is by no means the only theory of staining. A 

 recent discussion on the subject (Holmes, Stain Technology, vol. 

 iv, 1929, p. 75 ; Stearn and Stearn, Stain Technology, vol. v, 

 1930, p. 17) is of interest to those readers wishing to go deeper 

 into the subject. Much of the discussion relates to the question 

 whether to explain the phenomena on a chemical or physical basis. 

 The problem is complicated by the impossibility of distinguishing 

 completely between chemical and physical forces. Thus the 

 electrical theory outlined above and explained on physical terms 

 is not very different from the usual chemical theory, namely, that 

 basic dyes have an affinity for the acid j)arts of the cell, acid dyes 

 for the basic structures. The usual purely })hysical theory 

 assumes that dyes are fixed to the tissue by non -electrical adsorp- 



5—3 



