52 
power over the animals most susceptible to their influence. 
M. Arloing experimented with the anthrax bacillus and 
found that sun-light diminished the vegetative power of the 
mycelium. Two hours’ exposure to a July sun was suflicient 
to make a freshly infected broth sterile. Exposure for lesg 
than two hours retarded the vegetative power. When the 
spores are in process of actual development, however, the 
sun-light does not stop the growth ; the mycelium grows 
and produces spores, the filaments break up, and the spores 
are set free. The process is, however, slower and, in this 
respect, is analogous to the development as it takes place 
when the organism is cultivated in infusions which are little 
favourable to its growth. Sun-light, in fact, produces results 
analogous to those of culture in an unfavourable environ- 
ment. When the mycelium has already developed spores 
in a darkened stove, exposure to sun-light for thirty hours 
stops vegetation. The power of growth is gradually 
weakened by exposure to sun-light, before it disappears 
altogether. If a drop of a solarised culture is used as seed 
for a fresh infusion, the vigour of the second generation is 
diminished in direct proportion to the length of time during 
which the parent culture has been exposed to the solarising 
influence; the process of development is more and more 
protracted. Again, the diminution of vigour continues 
through successive generations. A third generation, the 
two preceding parent cultures of which have been exposed 
to the solarising influence, if itself exposed to sun-light, 
loses its vegetative power more rapidly than either of the 
two preceding generations ; the attenuative effect is, in fact, 
accumulated. These phenomena are accompanied by an 
attenuation of virulence if the successive cultures are inocu- 
lated in animals, and eventually the organism actually 
becomes its own vaccine. In proportion to the duration of 
the solarising influence and its continuance through succes- 
sive generations, a larger and larger quantity of the virus is 
