222 THE INCOME OF ENERGY 



Theory of Immunity. 1 Jenner discovered the 

 protective action of vaccinia against small-pox. The 

 small-pox virus when passed through a susceptible 

 animal becomes attenuated. This weakened poison 

 introduced into the circulation in man protects the 

 individual for long periods against the original disease 

 it establishes an artificial immunity against small- 

 pox. Schwann found that .fermentation and putre- 

 faction arose through the agency of micro-organisms 

 coming from without. Pasteur and Koch demonstrated 

 that the inoculation of animals with pure -cultures of 

 certain bacteria produced specific infectious diseases, 

 and that these cultures could be modified at will, 

 either by passing through the animal body, as in 

 Jenner's method, or in artificial culture media. Pas- 

 teur produced artificial immunity by using attenuated 

 virus. Behring discovered that the blood-serum of 

 animals immunized against diphtheria contained a sub- 

 stance which would protect other animals against the 

 toxine of diphtheria. So also with tetanus. Ehrlich 

 introduced the quantitative study of toxines and anti- 

 toxines by means of test-tube experiments, thereby 

 eliminating the uncertain factor of the animal body. 

 Thus it was shown in experiments on tetanus toxine 

 that the action of antitoxines is accelerated by heat, 

 retarded by cold, dependent on concentration in 

 short, that it is a chemical action. In the above ex- 

 periments on ricin, it is shown that the relation 



1 Ehrlich : Croonian Lecture, Proceedings of the Royal 

 Society, London, 1901, Ixvi, pp. 424-448. 



