THE DIFFERENTIAL ACTION OF DYES 



237 



same concentration. The blue dye is well retained, the red easily 

 washed off. 



MoUendorff ^^^ reached essentially the same conclusions from 

 independent studies. As he put it, from mixtures of two acid dyes, 

 the more diffusible goes into the more compact structures, the 

 more colloid into the more pervious. 



This subject was subsequently investigated in detail by Seki, 

 who noted the rate of penetration of dyes into agar gels contained 

 in test-tubes. In his experiments, of which full particulars are given 

 in his paper,^^^ the distances penetrated by certain acid dyes in 15 

 hours were these: — 



picric acid . 

 acid fuchsine 

 nigrosine W 

 methyl blue, 

 aniline blue WS 



25 mm 

 10 



7 

 5 

 4 



)5 



J> 



J> 



>) 



Details of a similar experiment are given in the Appendix (p. 

 322). The distances penetrated in 48 hours were as follows: — 



orange G 

 methyl blue . 

 aniline blue WS 



37 mm 

 16 



12 



n 



)) 



It will be noticed that the dyes that penetrate quickly are the 

 ones that colour the cytoplasm and those that penetrate slowly are 

 the ones that colour collagen, when suitable pairs are used. 



The fact that we are not concerned here with special chemical 

 affinities, but rather with the factors that have been mentioned, is 

 shown very clearly by the behaviour of acid fuchsine. This is an 

 intermediate kind of dye, for it is level-dyeing, yet moderately fast 

 to milling,^^^ and it penetrates at moderate speed. In Mallory's 

 technique ^^^ it is used with anihne blue, and here it colours the 

 cytoplasm; but in various techniques, of which Hansen's ^^^^ is 

 an example, it is mixed with picric acid, and now it colours the 

 collagen. It is clear that we are not concerned here with specific 

 affinities, but with the relative positions of the various dyes in a 

 scale of characters. The extremes in this scale are aniline blue WS 

 and picric acid. 



The dyes used for collagen mostly have higher molecular 

 weights than those used for cytoplasm (e.g. methyl blue 800, 

 picric acid 229). This, however, is not the direct cause of the 



