Book I CONTAGION 49

ment also, and then the combination called vinegar is produced;
which combination has its characteristic shape and distribu—
tion and disposition of parts, its charaecteristic taste and smell.
"That this process is preceded by a certain kind of putrefaction,
we see plainly in the case of milk, or phlegm, (mucus) for these
become acid at once when they are beginning to putrefy.?8 In
rabies also we must conclude that there is putrefaction follow—
ing contagion by some other body, but it is invisible to us, be—
eause the putrefactions that occur in the live animal are not
clearly manifest. But it is probable that this happens, for
dogs too, when they have rabies, are attacked by a sort of fever.
If therefore we proceed by induction, we shall conclude that
all contagions consist in some form of putrefaction. 'This is,
moreover, reasonable, since no other evaporation is more suit—
able for conveying contagions than that which occurs in putre—
factions.

I have said that the contagion lies in particles imperceptible
to our senses, and I may be asked whether these particles are
corrupted or only altered I reply that, in order to produce
putrefaction, it is not necessary that the particles themselves
be corrupted, but only altered in such a way that the combina—
tion is dissolved and evaporates the heat, together with the
innate moisture; not that there is anything to prevent their
being corrupted too, but that is not necessary for the produc—
tion of putrefaetion.

Now all putrefactions have the power to convey a precisely
similar putrefaction, at least to a continuous part; hence, if every
contagion is putrefaction, it seems that contagion, simply and
generally speaking, might be defined as?9 A cerfain precisely
similar putrefaction which passes from one thüng to another,
«vhether that other be continuous with the original thing, or sepa—
rated from it. Ket this is not contagion strictly so called, for
true contagion occurs between two different bodies. But if
we wish to consider, above all and by itself, that contagion
which is observed in diseases and does not affect by direct con—
tact only, then we shall define contagion as: — A precisely similar
putrefaction which passes from one thüing to another; its. germs
have great activity; they are made up of a strong and viscous com—
binafion; and they have not only a material but also a spiritual
antipathy to the animal organism. 'This definition will give us
the key to all the phenomena that are observed in contagion.