84 ESSAYS ON BACTERIOLOGY. 



ficially developed in one animal, could, by tlie trans- 

 fer of the blood, be transferred to anotber animal 

 wliich was previously susceptible. Bacteriology hav- 

 ing reached the stage of advancement that it had, it 

 did not take long to settle the fact that this series of 

 investigations had culminated in a most important 

 practical discovery. It was fully demonstrated that 

 a method, comparatively simple and apparently free 

 from harm, had been found, by which animals could 

 be protected against certain diseases, notably tetanus 

 and diphtheria, and could with equal certainty be 

 cured wlien those diseases had already entered the 

 body. 



The experiments upon human beings were then be- 

 gun, at first upon a small scale, then more extensively, 

 and with the results now known to all. This work 

 has been done by scientists in a scientific spirit, and, to 

 the ci'edit of scientific medicine be it said, there is no 

 secrecy and no degrading proprietorship in it. 



Speaking now more particularly of diphtheria, the 

 antitoxic serum is prepared as follows: Cultures of 

 the most virulent diphtheria germs are allowed to 

 grow in a fluid medium for several weeks. The fluid 

 is now found to contain the diphtheria toxin, an in- 

 tensely powerful poison. Small, non-fatal quantities 

 of this toxin are injected into an animal, usually a 

 horse. At inten^als, of some days, progressively in- 

 creasing quantities are injected, the result being the 

 development of a correspondingly progressive resist- 



