422 STUDIES IN IMMUNITY. 



phate to its original condition. The fact that the flocculation of 

 the complex by sodium chloride is reversible proves, however, 

 that this is not the explanation ; it is the entire complex which is 

 flocculated. 



Flocculation, on the contrary, is lacking when the complex, 

 barium sulphate plus colloid, has not been washed or when, after 

 it has been washed, an excess of stable colloid is added. This latter 

 colloid, then, would seem to "protect" complexes just as it protects 

 an unstable colloid from flocculation by salts. This fact, we think, 

 tends to validate the explanation of Beckhold, Girard-Mangan and 

 v. Henri, Pauli, and others.* They propose to explain the mechan- 

 ism of the protection of unstable colloids by stable colloids against 

 precipitation by electrolytes. When we remove a complex of 

 barium sulphate plus gum from the colloidal medium in which it 

 has formed, by washing, it is very probable that we do not obtain 

 it as it occurs in such a medium. If we accept the conception that 

 has been proposed by Van Bemmelen for the phenomena of adsorp- 

 tion, and admit that the preparation of emulsions of oil in gum 

 solution described by Quincke (that is, the covering of the oil 

 droplets by a layer of gum), is similar to the mode of prepara- 

 tion of our complex, we may suppose that the particles of the com- 

 plex, barium sulphate plus gum, are composed of a center formed 

 by the barium sulphate around which the particles of gum are 

 disposed in a homogeneous manner. The most distant of these 

 gum particles are attracted only feebly by the barium sulphate 

 and become detached in washing, which leaves us a complex 

 poorer in gum than in its original condition. We may then 

 logically imagine that on placing our washed complex in a fresh 

 gum solution it becomes covered with fresh exterior layers of gum 

 which have been removed by washing. The inhibiting effect of 

 gum solution on the flocculation of the complex by sodium 

 chloride may then be due to an increased thickness of gum and to 

 the intervention of additional superficial layers on the complex. 

 The following fact may be mentioned in support of this concep- 

 tion: if complexes of barium sulphate and gum are formed in 

 increasing quantities of gum solution and subsequently washed 

 in distilled water, we should obtain complexes of varying richness 

 * Cited by Aron, Bioch. Centralblatt, vol. 3. 



