148 LECTURES ON IMMUNITY 



lytic substance saponin into them, following which the red 

 blood-corpuscles are poisoned, so that they give up their 

 colouring matter, the haemoglobin. Now in a similar man- 

 ner the bacterial cells contain some substance that is a 

 good solvent for the corresponding agglutinin, which is 

 thereby caused to enter the bacteria to a preponderating 

 extent. The molecular weight of the agglutinin in the 

 bacterial solvent is only two-thirds of the molecular weight 

 of the agglutinin in the surrounding fluid, the physiologi- 

 cal salt solution. 



This behaviour of the agglutinin molecules in two sol- 

 vents recalls vividly the behaviour of benzoic acid in two 

 different solvents, water and benzene. According to 

 determinations of the freezing point of solutions of ben- 

 zoic acid, this has in aqueous solution the molecular weight 

 122, corresponding to the formula C 6 H 6 COOH; but in 

 benzene its molecular weight is double that. Therefore, if 

 we dissolve benzoic acid in water, and shake this solution 

 with benzene, the concentration of the aqueous solution C a 

 is related to the concentration of the benzene solution C b 

 as indicated in the following formula : 



where K is a constant factor. Nernst verified this equa- 

 tion by experiments on the distribution of benzoic acid 

 between water and benzene. 



The great velocity of absorption is in good agreement 

 with our interpretation as stated above. 



A difficult thing to explain is the specificity of the ag- 

 glutinins. The agglutinin produced by injecting typhoid 

 bacilli into the blood of an animal is only absorbed by 



