Vi PREFACE 



abandoned by most scientists. Although I adhere to the 

 chemical school, I cannot say that the proof of Ehrlich is 

 quite convincing, since the erythrocytes may well be 

 regarded as " living " even after their separation from the 

 blood of an animal. The further fact stated by Ehrlich, 

 namely, that approximately the #-fold quantity of a toxin 

 requires the n-fo\d quantity of antitoxin for its neutralisa- 

 tion, may be regarded as a more convincing proof against 

 the physiological hypothesis. The chemical hypothesis 

 is now generally accepted, and has been adopted recently 

 by Bordet, who originally expressed ideas similar to those 

 held by Metschnikoff. 



Nevertheless, many difficulties to the chemical hypothe- 

 sis remained. Nothing was more natural, therefore, than 

 that the further elucidation of the problem should be 

 sought through the aid of the modern theories of solution. 

 To this end Madsen and Ehrlich invited me to join in their 

 work. My work with Madsen in the Copenhagen Insti- 

 tute enabled us to fix upon a simple explanation of the 

 chief difficulty exhibited by the so-called phenomenon of 

 Ehrlich. The Chief of the Frankfort Institute was so 

 deeply interested in the progress of these studies that he 

 invited me to work in his Institute on the chemical be- 

 haviour of compound haemolysis. In this case, also, it was 

 determined that the laws of equilibrium found their appli- 

 cation. It would seem, therefore, that the adherents of 

 the chemical hypothesis should have felt wholly satisfied 

 with the results. However, one of the strange incidents 

 with which the history of science is replete occurred. In 

 our explanation of the investigated phenomena, especially 

 regarding the diphtheria toxin, Madsen and I, in accord- 

 ance with the usual rule in the exact sciences, tried to 

 employ as few hypotheses as possible, and in this we fol- 



