THE COLLOIDAL STATE 103 



In the absence of salts, it coagulates on heating to about 50, and becomes an 

 irreversible or suspensoid colloid. If neutral salts are present, the coagulum is 

 redissolved on boiling, owing, perhaps, to the formation of a chemical compound 

 with salts, although, as we shall see later, this interpretation is rather questionable. 

 If the suspensoid particles are given a positive or negative charge by traces of acid 

 or alkali, the precipitating effect of electrolytes comes into play, and the anion or 

 cation becomes prepotent, according to Hardy's rule. We have then a complex 

 state of antagonistic effects. There are two relations to be taken into account, one 

 between the salt as a whole and the protein molecule, perhaps a chemical one, 

 although the lyotropic effects described in the preceding section must not be 

 forgotten, the other relation, a physico-chemical, colloidal or electrical one between 

 the particle (qua particle) and the ions of the salt as carriers of electric charges. 

 In no other way can the experimental facts be satisfactorily explained. 



A few words are requisite at this stage as to the chemical nature of proteins, 

 so far as to make intelligible the way in which it intervenes in their colloidal 

 reactions. A more complete account will be found in Chapter IX. It has been 

 shown, mainly by the work of Emil Fischer ("Collected Papers," 1906), that these 

 substances are formed by the condensation of a number of molecules of various 

 amino-acids. Now the amino-acids are characterised by the presence of one or 

 more NH 2 groups, giving them basic properties, and one or more carboxyl groups, 

 giving them acidic properties. Alanine, or amino-propionic acid, is 



CH 3 -CH COOH 



NH 2 



They belong, therefore, to the class of electrolytes called by Bredig (1899) 

 amphoteric, behaving towards strong bases as acids, and towards strong acids as 

 bases. When the COOH and NH 2 groups are equal in number, as in alanine, the 

 amino-acid is very nearly equally strong as a base and an acid, and is therefore 

 practically neutral in reaction, actually very faintly acid. If the NH 9 groups are 

 in excess, as in the diamino-monocarboxylic acid, lysine, the substance becomes a 

 fairly strong base ; while, if the carboxyl groups are in excess, as in the mono- 

 amino-dicarboxylic acid, aspartic acid, we have a fairly strong acid. These acids 

 are capable of combining together by the COOH of one uniting with the NH 2 

 of another, with elimination of water, thus : 



becomes CO HN 



There are always some NH 2 and some COOH groups left uncombined, and 

 according to the relative number of these, the resulting protein or polypeptide 

 will have the properties either of a base, a neutral substance, or an acid. This 

 brief sketch will suffice for our present purpose, although it must be remembered 

 that some of the constituent amino-acids are complex compounds containing 

 aromatic, pyrrol, iminazol, etc., groups. 



When combined with base or acid, say sodium or hydrochloric acid, an 

 amino-acid forms a salt, thus alanine becomes sodium amino-propionate or alanine 

 hydrochloride respectively : 



CH, CH COONa CH 3 CH COOH 



3 | I 



NH 2 NH 2 HC1 



Similarly, the free NH 2 and COOH groups of the protein can react with acid or 

 base to form a salt. 



Now, like all salts, these salts of proteins are electrolytically dissociated in 

 solution, the sodium salt of globulin, for example, partially dissociates into Na- and 

 a large organic anion, which has the properties of the colloidal state. The 

 hydrochloride dissociates into 01' and a large colloidal organic cation. We see 

 thus how, by direct chemical means, we can obtain the same protein with a 

 negative or a positive charge. It appears, also, that these colloidal ions are very 

 ready to form aggregates, as the simple, insoluble, inorganic salts, such as 



