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Dr. W. M. Haffkine, 



There may exist already facts unknown to me, which are opposed to 

 the guesses implied in them. It was those guesses which led to the 

 results obtained in the plague inoculation ; but, in giving the reasoning 

 which I passed through while working out the method, I am yielding 

 only to a demand expressed to me to that effect, as I considered that 

 part of my communication unnecessary ; the more so that the theo- 

 retical conjectures above enumerated are not shared even by very 

 eminent experimenters, such as Pfeiffer himself, to whose results I owe 

 some of my premises ; and the correctness of the composition of the 

 plague prophylactic, with regard to the extracellular toxines which I 

 have added to it, the so-called supernatant fluid of the plague prophy- 

 lactic has been the subject of an animated dispute. 



It is certain that no theoretical views conceived by one experimenter 

 are binding, or need even be interesting, to others. What is obligatory 

 is the acceptance of the results obtained. 



The Plague Prophylactic. 



In order to accumulate for the plague prophylactic a large amount 

 of extra-cellular toxines, the bacilli are cultivated on the surface of a 

 liquid medium where they are suspended by means of drops of clarified 

 butter or of cocoanut oil. " 



The bacilli grow down in long threads into the depth of the liquid, 

 and produce what I have termed a stalactite growth in broth, an 

 appearance quite peculiar to this microbe, and which, I hope, will be 

 accepted as the specific diagnostic feature of this microbe. 



The products of their vital exchange — the toxines — are secreted by 

 the stalactites into the liquid and accumulate there. 



The growth is periodically shaken off the surface of the broth, after 

 which a new crop appears underneath that surface. 



Thus a large quantity of the bodies of microbes is collected at the 

 bottom of the cultivation vessel, and the liquid itself gets gradually 

 permeated with increasing quantities of toxines. 



The process is continued for a period of five to six weeks, at the end 

 of which the bodies of the microbes become extremely deteriorated. 



It will be seen from this that, in my eagerness to put to test our ability 

 to influence the case mortality I may have, perhaps, paid less atten- 

 tion than I might have done to the problem of reducing the number 

 of attacks; and I have now sketched out a simple plan whereby to 

 test this circumstance, and to try to improve our results from this 

 point of view. 



In order to render harmless the inoculation of the virus above 

 described, I deterr lined to kill the microbes by heating the material up 

 to 65 to 70° C. The virus so treated, differing from what is observed 

 in some other instances, loses at once, for the animals susceptible to the 



