The modern theory of solutions has had from the 

 beginning an intimate connection with physiology; 

 the investigations of Pfeffer and De Vries concer- 

 ning plasmolyse being one of the starting points for the 

 theoretical researches of Van't Hoff. Since then there 

 have been different adaptions of physical chemistry to 

 physiological phenomena. In this respect it will be suf- 

 ficient to mention the text books of Cohen, Koeppe and 

 Hamburger, giving a general summary of the investi- 

 gations in this field of work. 



It has not as yet been possible to introduce the 

 methods of physical chemistry into the theory of immu- 

 nity. This is probably due to the circumstance, that the 

 facts at our disposal have, up to the present time been 

 of very unequal value, the methods of investigation 

 being inevitably imperfect. Some of the most prominent 

 investigators of immunity have, as is well known, already 

 tried to interest physical chemists in their experiments; 

 but the researches being nearly exclusively of a qualita- 

 tive nature were not then sufficiently advanced for the 

 methods of physical chemistry to be applied to them. 



To Ehrlich belongs the honour of the first theoretical 

 foundation of the science of immunity, he having, through 

 the labours of many years, worked out exact methods 

 for the quantitative measurement of toxins and antitoxins. 

 When, by these means, we had gained a wide amount 



