242 DYEING 



methods is suggested by the fact that red blood-corpuscles 

 generally retain the acid fuchsine in such preparations. Mito- 

 chondria can also be coloured by the basic dye, crystal violet, 

 used hot.^^' ^^ 



Chemical affinity, density, and permeability play their allied or 

 antagonistic parts in the colouring of tissue-constituents, and it is 

 difficult to disentangle their effects. If, in any particular case, we 

 can be sure that there is no obstacle to penetration, and if we know 

 the chemical constitution of the substance that reacts with the 

 dye, and if further the substance is chemically homogeneous or 

 nearly so (unlike most proteins as they occur in the cell), we may be 

 able to estimate the amount of the substance present by measuring 

 the optical density of the dye taken up by it.^^-^ Thus a basic dye 

 may be used, at a pH too low to colour protein, to obtain an 

 approximation to the amount of DNA in a nucleus or of RNA in a 

 nucleolus or in the cytoplasm. A mordant-dye, chrome alum/gallo- 

 cyanine (p. 215), is particularly recommended for this purpose. *^^ 



