DISEASE-GERMS. 251 



The next important step in this investigation was the discovery of 

 the modification in the potency of the poison, which can be produced 

 by the " cultivation " of this bacillus. Every one knows that some of 

 our most valued esculent plants and fruits are the " cultured " varieties 

 of types which man would scarcely care to use in their original state, 

 on account of the unpleasantness of their flavor or their semi-poisonous 

 qualities. And, now that we know that these disease-germs are really 

 humble types of vegetation, the idea naturally suggests itself whether 

 they, too, may not be so far modified, by the " environment " in the 

 midst of which they are developed, as to undergo some analogous 

 modification. Two modes of such " culture " suggest themselves : the 

 introduction of the germs into the circulating current of animals of a 

 different type, and its repeated transmission from one such animal to 

 another ; and cultivation carried on out of the living body, in fluids 

 (such as blood-serum or meat-juice) which are found favorable to its 

 growth, the temperature of the fluid in the latter case being kept up 

 nearly to blood-heat. Both these methods have been used by Pasteur 

 himself and by Professor Burdon-Sanderson ; and the latter especially 

 by M. Toussaint, of Toulouse, who, as well as Pasteur, has experi- 

 mented also on another bacillus which he had found to be the disease- 

 germ of a malady termed "fowl-cholera," which proves very fatal 

 among poultry in France and Switzerland.* It has been by Pasteur 

 that the conditions of the mitigation of the poison by culture have 

 been most completely determined, so that the disease produced by the 

 inoculation of his " cultivated " virus may be rendered so trivial as 

 to be scarcely worth notice. His method consists in cultivating the 

 bacillus in meat- juice or chicken broth, to which access of air is per- 

 mitted while dust is excluded, and then allowing a certain time to 

 elapse before it is made use of in inoculation experiments. If the 

 period does not exceed two months, the potency of the bacillus seems 

 but little diminished ; but, if the interval be extended to three or four 

 months, it is found that, though animals inoculated with the organism 

 take the disease, they have it in a milder form, and a considerable pro- 

 portion recover ; while, if the time be still further prolonged, say to 

 eight months, the disease produced by it is so mild as not to be at all 

 serious, the inoculated animals speedily regaining perfect health and 

 vigor, f 



* I have seen notices of its serious prevalence during this very summer in some of the 

 localities most frequented by tourists. 



f It is not a little curious that as culture of one kind can mitigate the action of the 

 poison-germs, so culture of another kind may restore, or even increase, their original 

 potency. It has been found by Pasteur that this may be effected by inoculating with the 

 mitigated virus a new-born Guinea-pig, to which it will prove fatal ; then using its blood 

 for the inoculation of a somewhat older animal ; and repeating this process several times. 

 In this way a most powerful virus may be obtained at will a discovery not only practi- 

 cally valuable for experimental purposes, but of great scientific interest, as throwing light 

 upon the mode in which mild types of other diseases may be converted into malignant. 



