216 BIO-CHEMICAL JOURNAL 



necessary, in order for them to take up an electro-negative dye, that 

 the latter shall be discharged by the presence of a kation. When the 

 dye is electro-positive, on the other hand, neutral salts are unnecessary 

 and usually deleterious, since both classes of dyes are most sensitive to 

 the kation which would be inhibiting in the case of positive dyes. In 

 certain cases alkali increases the adsorption ot these latter, presumably 

 owing to the powerful action of the OH ion more than counteracting 

 the opposite effect of the kation. The common practice of using basic 

 dyes in strong solution of borax and acid dyes in acid solution is in 

 agreement with this view. It appears, that in the case of certain acid 

 dyes, such as congo-red, the concentration of the electrolyte used in 

 practice is unnecessarily high. 



5. The Staining of Histological Preparations 



Here again the balance of evidence is, as it seems to me, in favour 

 of the adsorption theory. 1 It may be that there are exceptional cases 

 where true chemical combination takes place, but they appear to be rare. 



This being so, it is obvious that the part played by electrolytes 

 must be taken into account. If electrolytes are split off from living 

 cells when they die or are injured it is clear why these cells readily 

 take up acid dyes under such conditions ; moreover, since electrolytes 

 are unnecessary when the substance to be stained is electro-negative, it 

 is also clear why living cells can be stained with basic dyes, if, as 

 seems probable, the structures in question have a negative charge. 

 The reaction of the tissues is on the alkaline side of neutrality and, as 

 Hardy has shown, globulin is electro-negative in alkaline solution, and 

 Pauli has shown the same for albumin. 



I am not forgetting the work of Overtoil on the solubility of 

 basic dyes -in the ' Plasmahaut,' and the insolubility of acid dyes 

 therein. The action of electrolytes must also, nevertheless, make 

 itself felt. 



An interesting case, which at the same time presents considerable 

 difficulty as to its explanation, is the staining of granules in the axis- 

 cylinder of nerve-fibres. These granules are stained by neutral-red as 



j. See Alfred Fischer, ' Fixierunjr, Farbung, etc. d., Protoflasmas.' Jena, 1899. 



