J 20 POPULAR SCIENCE MONTHLY. 



or exhausted by starvation. But, then, such a microbe when trans- 

 ferred into a fresh medium, if not dead, generally regains its vigor, and 

 after that, when inoculated into an animal, it produces its usual effect. 

 The remarkable circumstance about the culture left in the incubator 

 was that even when it was transferred into a fresh medium and its 

 vitality renewed, it remained still impotent. Pasteur concluded from 

 this that an infectious microbe possesses two distinct properties: one, 

 which it shares with .any other living being — viz., vitality — which may 

 be weakened or strengthened according to the conditions of life and 

 food; and another, which he considered as its 'virulence/ its power of 

 causing diseases, which may be also weakened or strengthened by special 

 means, but which is quite independent of 'vitality.' 



The lucidity of thought of which Pasteur made proof on this occa- 

 sion was magnificent. Later researches confirmed and explained these 

 facts with a singular completeness, and now the idea, as is always the 

 case, looks simple and self-evident. One must remember that at that 

 time Pasteur had every reason to believe that disease is caused by the 

 mere fact of a foreign micro-organism of a given species penetrating and 

 settling down to live in the system of a man or animal. Its capability 

 of living there, i. e., its vital properties, seemed all that was necessary 

 for causing disease. It was only later that it was found that pathogenic 

 microbes cause diseases by producing so-called toxines or poisonous 

 substances distinct from their own bodies and separable from them. 

 The process may be illustrated by a comparison, for instance, with a 

 cobra or any other animal producing a special venom. By starvation 

 or some other treatment the vitality of the cobra may be temporarily 

 weakened. When it obtains fresh food again and gets generally in good 

 condition, it recovers, without its ability of producing venom having 

 been in any way impaired. On the other hand, a snake may be by an 

 operation deprived of its fangs and power of secreting poison without 

 its health and strength being in the least affected. Pasteur at once 

 asserted that in a similar way it was possible by starvation to weaken a 

 breed of microbes without their virulence being diminished, and, on the 

 other hand, to deprive them of their power of producing disease without 

 impairing their vitality, though what the above power consisted in he 

 did not know. He called the latter result attenuation of a virus. An 

 attenuated virus in his meaning is therefore a special breed of patho- 

 genic microbes which can be maintained, by suitable breeding, in best 

 conditions of health, but which has lost either partially or entirely its 

 power of producing poison and disease. 



Pasteur extracted from the few experiments related above a further 

 most-important conclusion — viz., that such an attenuation was due to 

 and could be produced artificially by the effect of oxidation. This he 

 deduced from the fact that the microbes in the sealed-up tube had not 



