VACCINATION. 373 



twelfth day, which before had killed half the animals, there 

 was still only a slight febrile disturbance, and none of the inocu- 

 lated animals died. Virulent anthrax blood, might, after a 

 further interval of twelve days, be introduced into animals 

 that had been subjected to the double inoculation, without 

 giving rise to anything more than a slight febrile condition 

 similar to that noticed as resulting from the inoculation of 

 the modified virus. If, however, virulent anthrax blood was 

 introduced into animals in which only the first protective 

 inoculation had been made (i.e., with material that had been 

 cultivated for twenty-four days), a large proportion of the 

 animals died. It was evident, therefore, that it was abso- 

 lutely necessary to use both a first and a second vaccine if 

 the protection was to be complete. This attenuation was 

 not confined to the generations of bacilli that were directly 

 acted upon. If the temperature were lowered to about 35 

 C., vegetative activity was immediately set up, rods in 

 enormous numbers were formed, and eventually spores might 

 be observed in these rods. Now comes the interesting fact : 

 the attenuated properties of the original bacilli were handed 

 on to the spores ; these spores might be kept in a latent 

 condition for a considerable length of time, and on being 

 introduced into media suitable for their growth they sprouted 

 out, not into virulent anthrax bacilli, but into modified anthrax 

 bacilli, so that the conservation of the vaccine (on silk 

 threads) became a comparatively simple matter. 



Pasteur attributed the diminution of the virulence of the anthrax bacillus 

 to the action of heat in the presence of oxygen, but Chauveau, working on 

 Toussaint's plan, found that heat alone continued for a very short period was 

 quite sufficient to modify the virulence of the bacillus. Blood is taken from 

 a guinea-pig about thirty-six or forty-eight hours after inoculation with an 

 active virus ; it is carefully defibrinated and is run into small glass pipettes of 

 about i mm. in diameter. One end is carefully sealed by heat so far from 

 th blood that the heat cannot injure the organisms ; the sealed end contain- 

 ing the blood is plunged into water at a temperature of 50 C., and is kept 

 in this for about a quarter of an hour ; in this way is prepared what is 

 known as the primary vaccine, corresponding to that obtained by Pasteur, 

 by heating for twenty-four days. The second vaccine is heated for only 

 nine or ten minutes, and corresponds to that obtained by heat at the lower 

 temperature for twelve days. These vaccines are injected in the same way 

 as Pasteur's at intervals of from ten to fifteen days and they are found to 

 protect very fully against the most active virus. Where large quantities 

 of the vaccine have to be made by this rapid method, Chauveau used 

 sterilized broth which is inoculated with anthrax blood from a newly killed 

 animal ; the flasks are then kept at a temperature of 43 C. for about 



