OPTICAL AND ELECTRICAL PROPERTIES OF COLLOIDS 83 



reality a coagulation has occurred here, since the addition of a drop 

 of acetic acid or a little ammonium sulphate produces flocks. The 

 process is therefore called flocculation. These albumin flocks are in- 

 soluble in water. 



Only by the second process, by the agglomeration of the smallest 

 particles (see Plate I, b and c) under the influence of the acetic acid 

 or ammonium sulphate was it possible to separate the two phases 

 (water/albumin). This occurs in a way similar to that in which 

 the drops of rain are formed under certain conditions by the union 

 of particles of mist. The union is preceded by a slowing of the 

 Brownian-Zsigmondy movement, as has been shown with the ultra- 

 microscope by V. HENRI.* 



Flocculation is an electrical phenomenon. It is brought about by 

 electrolytes as well as by colloids of opposite electric charge, as well as 

 by ultraviolet and Rcentgen rays (the action of rays is much weaker 

 than the action of electrolytes). 1 If we shake purified lampblack 

 with water we get a suspension which remains turbid for weeks. 

 If we pour a few drops of an alcoholic mastic solution into water, 

 it remains milky for weeks and even years. We have seen that the 

 hydrophobe inorganic hydrosols, such as colloidal arsenic sulphid, 

 platinum sol prepared according to BREDIG'S method, gold sol pre- 

 pared according to R. ZSIGMONDY'S method, are permanent for 

 months provided the solution is free from electrolytes. Addition 

 of electrolytes causes an irreversible flocculation of these hydrophobe 

 colloids. 



The process is to be sharply distinguished from salting out, i.e., the 

 reversible precipitation of albumin, albumoses, etc., from solution by 

 large quantities of salt. 



The phenomenon of flocculation is encountered especially in the 

 precipitation reactions of albumins and in Chapter XIII on Im- 

 munity Reactions, where it plays an important role in the agglutina- 

 tion of bacteria by precipitins. Moreover the precipitation of gold 

 hydrosols by cerebrospinal fluid has acquired great significance in 

 the diagnosis of mental diseases (see p. 354). 



To cause flocculation, a certain minimum amount of electrolyte as 

 as well as of the dispersed phase is required. Below these limits, 

 which are characteristic for every electrolyte, no flocculation occurs 

 even after months. H. BECHHOLD called this minimum, the "elec- 

 trolyte threshold" and the "suspension threshold" 



The rate of flocculation is dependent on the concentration of the 

 suspension and of the electrolytes, i.e., the more concentrated the sus- 



1 Specific immunity reactions, in which precipitations occur, are probably not 

 electrical (see p. 197). 



