282 CHEMISTRY OF THE PROTEIDS CHAP 



alkali salts are of an intense blue colour. If one acidify the bku 

 solution of this salt with acetic acid till it has become colourless, anc 

 if one place a skein of silk in this colourless solution, it will stair 

 an intense blue. Many free colour acids, e.g. the sulphonic acids 

 of the amino-azo-compounds, differ in colour from their alkali salts, anc 

 as the free colour acids do not stain fibres with their own colour bui 

 that of their alkali salts, it follows again that the fibre must play the 

 part of a base. As a rule the fibre is unable to decompose the alkal 

 salts of strong colour acids, and therefore, the latter stain, only if b} 

 the addition of a stronger acid the colour-acid has been liberated." 



Martin Heidenhain (1902) has also carried out a very extendec 

 series of experiments in which he showed that most of the dyes usec 

 for microscopical work form coloured salts with the tissue-albumins 

 Salts are formed whenever* soluble or coagulated albumin, either in th< 

 form of a suspension or in the form of microscopical sections, is broughi 

 into contact with various dyes. He observed chemical changes whici 

 were quite as marked as when silver-oxide forms the insoluble silver 

 chloride on coming into contact with hydrochloric acid. He als( 

 noticed the amphoteric character of proteids, and gives the following 

 examples : The salts of the Nile-blue-base are blue while the free acic 

 is red ; on bringing together the red base with a solution of an albumir 

 there is formed the blue Nile-blue-albuminate in exactly the same waj 

 as if an acid had been added to the Nile-blue-base. Reversely the fre( 

 Congo-acid is blue, but it becomes at once red when a solution o 

 albumin is added. Solid proteids, e.g. microscopical sections, als( 

 become red on being placed in a solution containing the blue acid. 



The laws of hydrolytic dissociation hold, of course, good fo 

 coloured salts also. The acid dye-stuffs act best in acid media, for ii 

 neutral solutions the albumin-dye-compound undergoes hydrolysis, am 

 therefore, according to the strength of the colour acid, no or only ver 

 little staining takes place. An excess of acid, by preventing hydro 

 lysis, allows the albumin-dye-compound to remain insoluble, and there 

 fore the albumin appears coloured. Here again the behaviour of ai 

 albumin towards Congo-red is very instructive, for the Congo-red 

 albuminate will show the bright red salt colour, even if the solution ha 

 been rendered distinctly acid by the addition of acetic or of hydrc 

 chloric acid. The converse holds also good, for albumin coagula staii 

 blue in an alkaline, red solution of Nile-blue. 



In Heidenhain's papers will be found many examples illustratin, 

 the laws of dissociation. Some of the aniline dyes, particularly th 

 acid ones, form with albumins insoluble salts, and therefore precipitat 

 albumins as do alkaloidal reagents. (See above, p. 226, under Mathews. 



