Book I CONTAGION 15

in the pores of certain objects, so that they are not exposed to the
air or to alteration from outside, and, owing to the stzength
of the combination, they can hold out against many attacks.
Now & combination is strong and lasting in virtue of two quali—
ties; first it must have the kind of hardness possessed by iron,
stones and the like, whose very small, imperceptible particles
last for many years; secondly there must be present a certain
viscosity, and the mixing process must be thoroughly elabo—
rated. So that even when the germs of contagions are not hard,
they may be viscous and elaborated. By an elaborated com—
bination I mean one composed of very small particles well
shaken together. I described such combinations in my work
'On Sympathy'. Combinations of this sort are produced by
evaporations that are closely confined, where what evaporates
is not dispersed, but is violently shaken, and hence is very
finely and minutely mixed. Now if viscosity be added, the
resulting combination is strong and suitable for preservation
in fomes. A proof of this is that all germs that infect by means
of fomes are without exception viscous and sticky, and only
when they have this quality can they occupy fomes. Those
that are not viscous, but either have a dry basis per se, or abound
in water, or can alter very quickly, may, when they putrefy,
carry contagion to what they touch, but no fomes results, be—
cause either they do not adhere and stick, or they quickly alter.
This is the reason why decaying fruits infect by contaet, but not
by means of fomes, for they contain a great deal of water,
but what evaporates from them is not viscous. Viscous ele—
ments stick together and settle in what they touch, and since
they make a strong combination, they are not easily altered.
If they have analogy with the object touched, they at once
transmit the contagion; when they are not analogous, and the
object touched is not adapted to receive the infection itself
but is adapted to preserve the germs, then fomes is produced;
and if it presently encounters some body analogous with that
originally infected, it infects it precisely as the original would
have done. It makes no difference whether the contact is
direct or indirect. 8

Now all substances are not suited to become fomes, but
only those that are somewhat porous, and are warm or tepid.
In these, on account of their pores, the germs of contagions
can be stored up, and they cannot be altered either by the