INTRODUCTION 17 



substance may exert a great influence on the re- 

 action. Thus, for instance, agglutination of bacteria 

 does not occur in absence of salts and is also 

 prohibited by the presence of salts in higher 

 concentration. The first circumstance is analogous 

 to the sedimentation of suspended particles by 

 salts in solution, the second one is probably due 

 to the dissolution of the albuminous precipitate, 

 which causes the agglutination — strong solutions of 

 salts are good solvents for albuminous precipitates. 



Even if we try to avoid new hypotheses regard- 

 ing the presence of a great number of antibodies 

 (or antigens) as much as possible, we find that 

 it is characteristic that every antigen has its 

 special antibody, which does not react with other 

 antigens. This so-called specificity is of the 

 greatest importance, for it is possible to discover 

 an antigen amongst an immense number of other 

 organic substances by means of its specific antibody. 

 The blood of different animals, the secretions of 

 different bacilli may in this manner be discriminated 

 from each other with perfect certainty. In this 

 case ordinary chemical analysis leaves us absolutely 

 helpless. It may therefore be maintained that this 

 new department of science opens for us an immense 

 new field of chemistry of the very highest import- 

 ance to mankind. This circumstance explains the 

 exceptional interest of the investigation of this field. 

 As has been said above, it is physical chemistry 

 which gives us the mighty instrument for these 



investigations. This science itself has been greatly 



c 



