Book I CONTAGION 31

posed to violence, as I have said in my work 'On Sympathy'.!7
Hence the air keeps on dividing the evaporation more and more,
until it arrives at those parts which cannot be further divided
and separated. Then when this countless division has been
made, much of the air is filled and mingled with the evaporation
all round and about, as is most evident in the case of smoke.
"TThese then are the reasons why the evaporations that occur
in contagions are also carried all round and about, and occupy
a great volume of the air. For every exhalation has a great
tendency to disperse and to drift upwards, rather, at first, but
presently sideways, and finally down. 'Thus it is that these
germs may infect also those who live with persons infected,
and the germs can be preserved for a certain time, not only in
fomes but also in the air, though longer in fomes.

But how does it happen that germs whose bulk is so small
do not suffer alteration, when thus exposed to the air? That
is the first question. What must be the strength of the com—
bination in so small a particle, especially when those particles
have not the quality of hardness, that they can last so long in
the air? " It is those that are viscous and sticky, however small
they may be, that can live, if not quite so long as the hard ones,
still nearly as long.

'"The hard particles offer most resistance to alterations, be—
cause of three properties: First, in a small bulk they have
more substance; secondly, they are colder, on account of their
earthy elements; thirdly, by reason of their density, their
parts cannot easily be volatilised and rarefied as ought to hap—
pen when heat is introduced. 'Though viscous particles do not
possess the two first properties to such a degree as hard particles,
nevertheless they too have these properties in part. And they
have the third property just as much as do the hard particles,
I mean that they are not easily volatilised, because of the te—
nacity of their parts, also their combination consists of the
very smallest elements. For tenacity makes it hard for one
part to separate from another; and a combination made up
of the very smallest particles, well mixed, can resist all altera—
tions, since, at the threat of rarefaction, there are at once at
hand earthy parts next to every tiny particle, and these pre—
vent rarefaction. And when condensation would naturally
result, there are present fiery parts to resist in a similar way,
by reason of their nearness. 'That is why not only hard, but