vi ALBUMIN-DYE COMPOUNDS 229 



As these precipitates are deeply coloured they serve as delicate tests 

 for the detection of albumins. 



Attention has already been drawn to the fact that albumins differ 

 from one another chemically. As all albumins are both basic and acid 

 in their character, they must unite with whatever colour is offered to 

 form coloured salts, and one finds indeed in most cases that tissues 

 are stained at first in a diffuse manner, but that the stain is not uni- 

 formly resistant to after-treatment. If, for example, we have used a 

 basic colouring matter, such as methylene-blue, then on washing the 

 tissue, the salts which methylene-blue has formed with the feebly acid 

 (more basic) albumins will become hydrolysed to a greater extent than 

 will the salts which have been formed with decidedly acid albumins. 

 The more basic albumins becoming hydrolysed part with the methy- 

 lene-blue, and so become colourless, while the acid albumins form in- 

 soluble, ' permanent ' compounds. If flocculi of coagulated neutral egg- 

 albumin and of acid casein are suspended in alcohol, if Congo-red and 

 Nile-blue are added simultaneously, if the solution is rendered alter- 

 nately acid and alkaline, and if the coagula are well washed out, the 

 casein will be found to have stained blue with Nile-blue, while the egg- 

 albumin is stained red by the Congo-red. Fixation coagulates the 

 tissues, but does not alter their chemical character (see Chapter VII.); 

 if anything the differences between various proteids are accentuated 

 by the use of acid fixatives or of formaldehyde, which weakens the 

 basic character. Maschke l in 1859 was the first to employ carmine 

 and indigo carmine, besides iodine, for staining albuminous substances. 



The tissue constituents which stain most readily with basic dyes 

 are the nuclei, mucus, cartilage, amyloid, and yolk constituents, for 

 they contain strongly acid radicals in the nucleic acids, nucleo-proteids, 

 mucins, mucoids, and vitellins. 



Space forbids to discuss the chemistry of the hardening and staining 

 methods employed in histological research. These questions have been 

 dealt with for the first time in a systematic manner in the author's 

 book,' 2 after previous attempts (Heine 3 ) had met with difficulties. 

 There cannot be any doubt that the elective staining of tissues depends 

 on differences in the ba'sicity and acidity of the proteid substances. 



To say that staining depends on differences in the coefficient of 

 distribution means nothing, for the question we have to answer is 

 what causes the difference in the coefficient, and this is undoubtedly 

 due to differences in the chemical nature of the two media, amongst 

 which the dye distributes itself. (The author's Physiological Histology.) 



1 0. Maschke, Botanikerzeitung, 17. 21 (1859). 



2 Gustav Mann, Physiological Histology, Methods and Theory, Oxford, 1902. 

 3 L. Heine,- Zeitschr. f. physiol. Ghem. 21. 494 (1896). 



