34 THE POPULAR SCIENCE MONTHLY. 



the purpose of vaccination might be obtained by using the guinea- 

 pig as an intermediate host. Greenfield experimented also at the 

 same time and in the same direction with very satisfactory results. 

 In 1883 Pasteur and Thuillier made experiments in the rouget des 

 pores, showing clearly that by causing the virus to pass through 

 a series of pigeons it became gradually more virulent for the pig ; 

 while the reverse was true when series of rabbits were used as 

 intermediates, showing thus at the same time how disease may 

 originate or be modified. The principle of vaccination as intro- 

 duced by Jenner seemed therefore to have become a fact capable 

 of general application by this time. All that seemed necessary 

 was to find a suitable intermediate host for the parasite. By 

 growing the parasite in a series of such intermediate animals one 

 was able to obtain a " vaccine " or " lymph/' which, like that of 

 vaccinia, was capable of inducing immunity, as well as an attack 

 of the unmitigated disease would have, but without the same 

 danger. Another fact of great importance has also been noticed 

 by several observers viz., that the occurrence of certain febrile 

 affections may modify the course of other febrile affections previ- 

 ously established. Daniellsen and Boeck were perhaps the first 

 to notice this fact, and to record the beneficial influence which an 

 attack of small-pox might have on the course of tubercular lep- 

 rosy. I need not refer to the numerous attempts which have been 

 made since in that line. 



Refractory State produced by Inoculation of a Virus modi- 

 fied by Cultivation outside the Body. Another discovery, more 

 fertile in results, although similar in principle, was, however, 

 made known by Pasteur in 1880. He showed that by cultivating 

 the microbe causing chicken cholera at a temperature of 33, with 

 free access of oxygen, he could obtain in from two to eight months 

 an attenuated virus, which would cause the disease in a form 

 mild, but sufficient to produce immunity. Toussaint and Chau- 

 veau were experimenting at the same time on the anthrax bacil- 

 lus. Toussaint made the important discovery that, by heating 

 cultivations of the Bacillus anthracis rapidly to a comparatively 

 high temperature, its virulence was attenuated. In 1881 Pasteur 

 produced attenuation of virulence of the Bacillus anthracis by 

 cultivation for nine days at a temperature of from 42 to 43 C, 

 and produced immunity by vaccination with the modified virus. 

 Koch, Gaffky, and Loefner repeated Pasteur's experiment in 1884, 

 confirmed his results, and extended them, differing from him only 

 with regard to the influence which he attributed to oxygen. In 

 1882 Pasteur and Thuillier applied to the bacillus of "rouget" the 

 same principle as that used in the case of the chicken cholera and 

 splenic fever, and succeeded in attenuating that virus also. Dur- 

 ing the whole of this period Pasteur was occupied in trying to 



