THE CAUSES OF DIFFERENTIAL DYEING 99 



phosphoric groups. The acidity of the mucopolysaccharides of 

 cartilage and of certain mucous secretions is due to sulphuric 

 and carboxyl groups. The amphoteric proteins owe their char- 

 acter to a balance between acidic amino-acids (aspartic, glutamic, 

 hydroxyglutamic, &c.) and basic ones (lysine, arginine, and 



I 

 NH 



I 

 HC(CH2).,COOH glutamic acid 



i 



c=o 



I 



NH 

 I 

 HC(CH2)4NH., lysine 



I 



c=o 



I 

 Part of a protein chain at the iso-electric point 



histidine). The latter predominate in the basic proteins of 

 collagen and red blood-corpuscles. 



When a protein is put in a fluid at a pH below its iso-electric 

 point, the amino-groups of its basic amino-acids tend to become 



positively charged as -NH3. When the pH is above the iso- 

 electric point, the carboxyl groups of the acidic amino-acids 

 become ionized as -COO~. If the proportions of basic and acidic 

 amino-acids in a particular protein are about equal, the electric 

 charge of the protein as a whole is easily shifted by variation in 

 pH within the range of the fluids ordinarily used in microtech- 

 nique; but in fact all proteins can be shown to be amphoteric if 

 there is a sufficient change of pH. 



We have seen in chapter 7 that all dyes dissolve as ions, and 

 that the coloured ions of some are positively charged, of others 

 negatively. This can easily be proved by experiment. An agar gel 

 is allowed to set in a vertical U-tube, and the dye-solution to be 

 tested is added to both limbs of the tube, above the agar. The 

 agar serves the mechanical function of holding the water in the 

 U-tube nearly still. Salt must be dissolved with the agar, to permit 

 the easy passage of an electric current. An electrode is placed in 



