124 STAINING. 



cations, strongly adsorbed by the negative paper, but the free base 

 is also. Since foreign electrolytes dimmish 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 ejffiect of 

 alcohol is in the same direction as that of electrolytes, sincejt^also 

 decreases the electric charge and, therefore, 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 probably 

 accounts 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 und Ban des Protoplasmas, Jena, 1899 ; PELET- 

 JOLIVET'S Theorie des Farbeprozesses, Dresden, 1910 ; First Report 

 on Colloid Chemistry, Brit. Ass., 1917 ; GEE and HARRISON, Trans. 

 Faraday Soc., vol. vi, 1910 ; HARRISON, Journ. Soc. Dyers and 

 Colourists, December, 1911 ; BAYLISS, Biochem. Journ., vol. i, 1906, 

 p. 175. 



The reducing properties, in the chemical sense, of cell constituents 

 play an important part in certain special staining reactions, particu- 

 larly those with metallic salts. Osmic acid (osmium tetroxide) is 

 reduced to a lower oxide or perhaps to the metallic state by un- 

 saturated fats. Gold and silver salts are reduced by nervous tissue 

 under appropriate conditions and deposited in the colloidal state. 

 All the various colours of colloidal gold in different degrees" of 

 dispersion may be seen in tissues prepared by the gold impregna- 

 tion methods. Details of these methods will be found in other 

 chapters of this book. 



Effect of Heat. Since surface tension is decreased by rise of 

 temperature, it would naturally be expected that, if this factor is 

 concerned in the process of staining, the depth of the stain would be 

 lessened at a higher temperature. This is actually the case with 

 simple substances like cellulose up to 50 C. or thereabouts. At 

 first sight this would seem to be at variance with the frequent 

 practice of fixing stains by heating the preparation. But the 

 temperatures used are much higher than those referred to, and, in 

 point of fact, if filter paper is stained in a Congo red solution at 100 C., 

 the dye is firmly fixed, and cannot readily be removed by washing. 

 It is difficult to say what actually happens here. Chemical combina- 

 tion suggests itself, but the nature of the compound formed is not 

 easy to conceive. The temperature at which a material possesses 

 the maximum electric charge, as shown by GEE and HARRISON, also 

 plays a part in the phenomena. 



