vin EFFECT OF ALTERING ELECTRICAL TENSIONS 279 



complex-ions, or, as he put it in 1902, in his Physiological Histology, p. 

 46, " If the aggregates in a colloidal solution are completely broken up 

 into the composing units by acquiring definite charges, then the colloi- 

 dal solution loses its colloidal character and becomes a solution of a 

 completely dissociated electrolyte. Reversely, if to a solution contain- 

 ing a definite number of electro-positive (colloid + H) -ions there is 

 added an alkali containing the same number of electro-negative 

 hydroxyl-ions, then the H of the colloid and OH' of the alkali unite 

 to form electrically neutral water, and the colloid having lost its 

 electrical charge is precipitated ; if, however, not a sufficient number 

 of OH'-ions were added to bind all the hydrogen-ions, then the colloid 

 aggregates rearrange themselves into larger aggregates, which will 

 remain ' in solution ' as long as the H-atoms, joined to the colloid, 

 enable its aggregates to maintain a definite charge. As the colloid 

 particles become larger and larger and the number of electrical charges 

 for a given volume of solution fewer and fewer, complete precipitation 

 will probably be induced at a time short of perfect neutralisation of 

 the charges, because of the specific gravity of the colloid-aggregates 

 .overcoming the viscosity of the fluid." The same principle has recently 

 been put forward by Billitzer, 1 who, without accounting for the reason 

 why colloids possess their original charge, simply states that the pre- 

 cipitating ion is carried down by colloids in such quantities as have 

 sufficed to electrically neutralise the original charge on the colloidal 

 particles: " Because an ion carries a load, which is considerably in excess 

 of that carried by the suspended particles of the colloid, an electrical 

 neutralisation of colloidal particles can only occur, if so many colloidal 

 particles aggregate round an ion as to neutralise its charge. Hereby 

 complexes of large diameter are formed which are thrown down along 

 with the precipitating ion whenever the critical point has been reached 

 at which the forces of gravitation make themselves felt. The ions 

 form condensation-nuclei round which the suspended particles aggre- 

 gate." Billitzer also speaks of true and false colloids, the former he 

 believes to be precipitated by whatever electrolyte one adds, while 

 false colloids are characterised by being only precipitated by one set of 

 electrolytes, while others instead of causing a precipitation may lead to 

 greater subdivision of the particles. As examples of false colloids are 

 mentioned especially gelatine and egg-white. 



The division into true and false colloids is quite untenable, 



Billitzer has not taken into account that the coagulation of his ' true ' 



electrolytes is brought about by the difference in the rate of diffusion 



of the ions of the electrolyte which is added, nor has he in the 'false' 



1 J. Billitzer, Zeitschr. f. physik. Chem. 51. 133 (1905). 



