ix PROTECTIVE INOCULATION 277 



cultures from the same source ; whereas greater uniformity- 

 is to be anticipated from material derived from the same 

 breed of guinea-pigs, all animals being of about the same 

 weight and inoculated with material of like activity. 



There remains to be determined the interesting and 

 important question : Whence is derived the especial 

 efficacy of the new prophylactic, which contains, be it 

 remembered, not only the dead bodies of plague bacilli 

 and associated tissue- toxin, but also, in all probability, 

 other tissue constituents? In this connection I have 

 made experiments which prove that the clear filtrate from 

 a watery emulsion of the prophylactic powder possesses 

 undoubted prophylactic efficacy. Thus, injection of an 

 amount of clear filtrate corresponding to, i.e. obtained from, 

 20 to 25 milligrammes of dry powder, confers immunity 

 on the adult rat against a subsequent cutaneous infection 

 with virulent B. pestis. This, of course, shows that the 

 new prophylactic does not act solely by means of the 

 bodies of the dead bacilli which it contains. 



These experiments also show, as might be expected, 

 that the whole organs prophylactic — i.e. the entire powder 

 — is more efficacious than the watery extract, since of a 

 definite sample of the organ prophylactic, of which 15*6 

 milligrammes of the entire material is sufficient to protect a 

 rat, 20 to 25 milligrammes are required to yield an equally 

 efficacious watery extract. Eoughly speaking, the pro- 

 portion between a given entire prophylactic and its watery 

 extract (filtrate) is as 15 to 25. 



A further important point ascertained was this, viz. 

 that the filtrate of a watery emulsion (filtered simply 

 through filter-paper) can be sterilised by heating it to 

 70° C. for fifteen minutes, and that even twice so heated, 



