2£6 REPORTS ON THE STATE OF SCIENCE 



Proteins become charged by the electrolytic dissociation of the 

 hydrogen of the carboxyl group. Their salts, with acids and bases, 

 are also electrolytically dissociated. In the case of the salts with 

 acids the charge carried by the protein is of opposite sign to that caused 

 by the dissociation of the hydrogen of its carboxyl groups, so that 

 the protein may be less or more positively charged, according to the 

 degree to which salt formation has occurred (the acidity of the solu- 

 tion), or, if the two effects balance, uncharged (iso-electric point). 



At the iso-electric point the maximum surface tension between 

 particle and liquid and the minimum repulsion between particle and 

 particle will exist, hence this is the optimum point for agglutination. 

 In the presence of alkali (H+ concentration less than 10- 7 N) agglu- 

 tination cannot occur, as the negative charge on the protein is sufficient 

 to keep the particles apart. 



Before attempting to explain the peculiar effect of temperature 

 on agglutination, we must mention that we could only study this 

 effect under circumstances which were not entirely favourable to 

 agglutination, i.e., when it took place slowly. In our experiments the 

 surface tension was small, and the particles were always charged, 

 and therefore repelled one another. In order that this small surface 

 energy may be effective, the particles must be brought within a very 

 small distance of one another. The necessary translation is effected by 

 their own intrinsic energy (Brownian movement), but until the tem- 

 perature rises to a certain point the mean velocity of the particles is 

 insufficient to overcome the repulsion due to the charges. Once that 

 temperature at which some of the particles possess the requisite 

 velocity is exceeded, the effect of temperature is for the time being 

 very great owing to the way in which the velocities of the individual 

 particles are distributed about the mean, which in this case we may 

 suppose to be normal. At temperatures higher than that at which 

 all, or nearly all, the particles possess the requisite velocity, tempera- 

 ture will influence agglutination rate only to the degree to which it 

 erdianees mean molecular energy, viz., two to three times per 10° C. 

 rise. 



