STUDY OF MOLECULAR ADHESION. 419 



is one of those most strongly indicative of the relation which would 

 seem to exist between adsorption phenomena and the reactions 

 between agglutinins and bacteria. 



The suspension of an inorganic powder in a colloidal medium 

 begins, then, by adhesion of the colloid to the powder. We have 

 just seen that certain colloids on uniting with barium sulphate 

 agglutinate it. The two phenomena of agglutination and dissocia- 

 tion of barium sulphate by stable colloids, although apparently so 

 different, are in fact due to the same mechanism, namely, the adhe- 

 sion of the colloid to the suspension. 



We have already found that a given colloidal substance in uniting 

 with a given suspension may produce either its dissociation or its 

 agglutination. This is particularly the case with such colloids as 

 gelatin, the concentrated solutions of which harden in the cold. 

 The dissemination or the agglutination of a suspension of barium 

 sulphate by a 0.5 per cent solution of agar depends on whether 

 the mixture is made at 50 or at 16 C. In both instances 

 adhesion takes place. It is probable that weak solutions of agar 

 at high temperature are composed of very small particles of the 

 substance, which particles clump together as the temperature is 

 lowered. This supposition indeed finds an experimental basis 

 in the facts that Hardy* has noted. This author found that solu- 

 tions of agar and gelatin are composed of granules, the volume and 

 disposition of which depend on the temperature. The effect of 

 agar, then, on barium sulphate depends on its physical condition 

 or on the size of its particles, which latter fact depends on the 

 temperature and the duration of time that the colloid has remained 

 at this temperature. 



The effect of cold on agar corresponds to the effect of heat on the 

 protein substances of serum; these substances when unheated are 

 in a colloidal condition which permits them to adhere to barium 

 sulphate and to disseminate it; when larger proteid particles have 

 been produced by heating the clumping of the sulphate follows.! 



These facts, we believe, may be compared with another phenom- 

 enon : if a fresh amount of barium-sulphate suspension is added to 

 a complex of washed barium sulphate and colloid (for example, 

 barium sulphate plus gum arabic), complete agglutination of the 



* Proceedings of the Royal Society, 1900, LXVI. f Gengou, p. 326. 



