Book I CONTAGION 17

fomes itself or by external factors, unless these are excessively
active; for instance they have no defence against fire.^  It fol—
lows that iron, stone and bodies of that sort, which are cold
and not porous, are not adapted to become fomes, whereas
wool, rags, and many kinds of wood are well adapted.  From
what has been said it is evident that contagions that infect
by means of fomes and those that infect by direct contact only,
have a common principle and a common method of convey—
ing the infection. But they do differ in the kind of combina—
tion of the germs, since in the former case the combination is
strong and viscous, but in the latter it is weak and not viscous.
"This is the reason why some germs produce fomes, while others
do not.