THE COLLOIDAL STATE 107 



adsorbed, and therefore inactive, substances may be set free to manifest their 

 activity, or "mobilised," to use a frequent form of expression. 



The proteins of blood plasma do not appear to serve as food to the tissue cells. Quagliariello 

 (1912, p. 174) showed that, when injected into the blood vessels, they are only utilised with 

 extreme slowness. It appears that their chief value is due to their properties as colloids. 



COMPLEX COLLOIDAL SYSTEMS 



When a solution of an electro-positive colloid, such as ferric hydroxide, is 

 added to one of an electro-negative colloid, such as arsenious sulphide, if the 

 proportion of the two is such that the charges will mutually annul each other, 

 both colloids are precipitated as a complex, and the solution is left free from 

 both. This phenomenon has been investigated especially by Biltz (1904). The 

 precipitate will, in such a case, have no charge. If excess of either colloid 

 is present, only partial precipitation will occur, and both colloids will be 

 present in the precipitate and in the liquid above, although in different pro- 

 portion in the two. In other words, we have an adsorption compound formed, 

 whose composition depends on the relative concentration of its components in 

 the solution, and whose electric charge has the sign of that colloid which is 

 in excess. The fact that only partial precipitation or mere aggregation takes 

 place when either colloid is in excess is sometimes put in the form that the 

 precipitate is soluble in excess of either colloid. 



In such a comparatively simple system we see already conditions of much 

 complexity, and when, in addition, emulsoid colloids, or proteins, are present, the 

 possibilities are still more manifold. 



The triple adsorption compounds of Raehlmann (1906) have already been 

 described (page 65), and one or two examples of other complex systems may be 

 instructive. 



The fact that filter papers take up a greatly increased amount of Congo-red 

 when its negative charge is reduced or reversed by cations, such as Ca - , has been 

 previously referred to. Now, if gelatine be added, this effect is practically 

 abolished, because the gelatine coats the paper with an emulsoid, itself insensitive 

 to Ca 1 ions. Egg-white behaves in the same way as gelatine, if in neutral 

 solution ; but, if made acid (i.e., electro-positive), it increases the action of Ca - ions, 

 and if alkaline, it diminishes their action, as in neutral solution (see Bayliss, 

 1906, p. 201). 



The following experiment of Larguier des Bancels (1908, p. 198) is of interest 

 in showing how an effect varies according to concentration : 2 c.c. of a dilute 

 (0'125 per cent.) solution of aniline blue is totally precipitated by 5 drops of a 

 certain ferric hydroxide preparation. If 5 drops of saturated ammonium sulphate 

 be added in addition, only partial precipitation occurs ; the solution is left deep 

 blue. But if 40 drops of the ammonium sulphate be added, the precipitate is 

 again nearly total. 



As cases where we have to deal with complex mixtures of interacting colloids, 

 we may mention : the coagulation of the blood, and the innumerable phenomena 

 connected with haemolysins, immunity, and anaphylaxis, together with intracellular 

 processes in general. 



An elaborate system of names has been introduced, especially in connection with 

 haemolysins and the coagulation of the blood, names which imply definite chemical individuals. 

 From the complexity of the results to be obtained in colloidal reactions, from a very few 

 distinct chemical substances, it seems more than probable that, as soon as sufficient 

 knowledge is obtained of the nature of the phenomena in the systems referred to, the 

 necessity for most of these names will be found to have disappeared. At present we find an 

 investigator content to refer an experimental result to, say, "deviation of complement," 

 apparently unaware that he is merely translating into a classical language what he has 

 previously described in his mother-tongue. This particular case appears to be merely 

 one of adsorption, a general phenomenon explicable on such fundamental laws as the 

 principle of Clausius and Carnot. This question of terminology will be of necessity 

 mentioned again (pages 307 and 328 below). 



The papers of Gengou (1908) will be found very instructive in connection with the 

 subject of the present section. 



