84 
Pasteur and his PFork, 
successive cultivation of them in an artificial medium, and in 
as perfect a vacuum as possible, or in contact with carbonic 
acid gas, but no air, as this kills them — conditions the opposite 
of those in which the anthrax spores develop. From blood con- 
taining these two organisms of anthrax fever and putrid fever 
(septicaemia), he could produce either disease experimentally, 
at will, according as he cultivated the germs with or without 
air ; or by inoculating with blood from an animal just dead 
from the former malady, or obtained from it twenty-four hours 
after death. 
These discoveries clearly established the correlation between 
the phenomena of fermentation and those of contagion. There 
is no longer anything mysterious about the latter. It is simply 
the breeding of a living organism, or "element," in a living 
body, and at the expense of that body ; in the same way as the 
organism of fermentation lives and m.ultiplies in, and at the 
expense of, the dead organic matters into which it has found 
its way. Just as the contagious itch, or " mange," is produced 
on the surface of the skin by a very small insect or mite (the 
Acarus scahei) ; and " ringworm," also contagious and situated 
on the skin, is due to a microscopic vegetable fungus (the 
Tricopliyton tonsurans) ; so are what we may term the " internal 
contagious disorders," some of them comparatively mild, as 
foot-and-mouth disease ; and others most fatal — as anthrax, 
cattle-plague, and sheep-pox — due solely to living germs which 
prey upon the bodies of the animals they invade, and for 
whose fluids and tissues they each have a special predilection. 
I shall recur to this interesting feature presently. In the 
meantime, it may be stated that the question as to whether 
contagious diseases ever arise spontaneously is settled in the 
negative : spontaneous contagious disease has no more founda- 
tion than the spontaneous generation of organised bodies ; and 
I need not discuss the primary origin of these diseases or the 
germs which occasion them, any more than I need do the 
origin of potatoes, turnips, dogs, or horses, when studying the 
attributes, derivation, dr natural history of these or any other 
vegetable or animal body. 
When studying splenic fever, Pasteur was struck by the fact 
that the microbe which causes it is under a particular influence 
which prevents it developing everywhere, and limits it to cer- 
tain media which, one would think, do not difl'er from other 
media in any appreciable manner. Thus in the ox, sheep, 
rabbit, guinea-pig, and some other creatures, it is readily 
inoculable and terribly fatal ; but the dog and pig have to be 
inoculated several times before they are infected, and even not 
then with certainty ; while fowls are proof against the disease. 
