THE DIFFERENTIAL ACTION OF DYES 239 



cytoplasm, and red blood corpuscles in three different colours; 

 for instance, with aniline blue, acid fuchsine, and orange G 

 respectively in Mallory's technique. 



The fact that it is so particularly easy to colour collagen and red 

 blood corpuscles differently is interesting. Both of these are ex- 

 amples of strongly basic substances, for collagen contains a high 

 proportion of arginine and lysine, and the globin of haemoglobin is 

 rich in these and in histidine. One would therefore expect them 

 both to be strongly acidophil. So indeed they are; but red blood 

 corpuscles are so impermeable that the most powerful milling 

 dyes can scarcely enter them. Thus in the differential action of 

 acid dyes, physical or mechanical factors predominate over 

 chemical affinities. 



Some of the powerful but slowly diffusing dyes used for collagen 

 colour collodion in acid solution. *^^ Thus aniline blue and methyl 

 blue dye it strongly from pH2 to PH5. This is unusual behaviour 

 for acid dyes. One would expect the negatively charged anion to 

 be repelled by the similarly charged substrate. These two dyes 

 have some capacity to act as though they were basic, and indeed 

 aniline blue possesses an amino-group. Once again acid fuchsine 

 is intermediate between these and the levelling dyes, for it colours 

 collodion moderately from pHz to pH6. 



Induline colours collodion less strongly at low pH, because it 

 tends to flocculate. 



When it is desired to colour collagen differently from other 

 tissue constituents, use is often made of phosphomolybdic acid. 

 The techniques employed are variants of the procedure intro- 

 duced in 1900 by the American histopathologist, Mallory.^^^ In 

 his technique sections were treated with an aqueous solution of 

 phosphomolybdic acid and then with a mixture of aniline blue, 

 orange G, and oxalic acid in water. The oxalic acid served simply 

 to lower the pH and thus help the action of the levelling dye, 

 orange G. (The dyeing of the chromatin is irrelevant and will not 

 be considered here.) 



Molybdenum is a metal related to chromium. The yellowish 

 white oxide, M0O3, insoluble in water, dissolves in ammonia 

 solution to produce ammonium molybdate (p. 294), which reacts 

 with orthophosphoric acid, to produce ammonium phosphomoly- 

 bdate; this, when dissolved in aqua regia, deposits pale yellow 



