to be cut.
A thing is viscous when, being moist or soft, it is tractile. Bodies
owe this property to the interlocking of their parts when they are
composed like chains, for then they can be drawn out to a great length
and contracted again. Bodies that are not like this are friable.
Bodies are compressible when they are squeezable and retain the
shape they have been squeezed into; incompressible when they are
either inapt to be squeezed at all or do not retain the shape they
have been squeezed into.
Some bodies are combustible and some are not. Wood, wool, bone are
combustible; stone, ice are not. Bodies are combustible when their
pores are such as to admit fire and their longitudinal pores contain
moisture weaker than fire. If they have no moisture, or if, as in
ice or very green wood, the moisture is stronger than fire, they are
Those bodies give off fumes which contain moisture, but in such a
form that it does not go off separately in vapour when they are
exposed to fire. For vapour is a moist secretion tending to the nature
of air produced from a liquid by the agency of burning heat. Bodies
that give off fumes give off secretions of the nature of air by the
lapse of time: as they perish away they dry up or become earth. But
the kind of secretion we are concerned with now differs from others in
that it is not moist nor does it become wind (which is a continuous
flow of air in a given direction). Fumes are common secretion of dry
and moist together caused by the agency of burning heat. Hence they do
not moisten things but rather colour them.
The fumes of a woody body are called smoke. (I mean to include bones
and hair and everything of this kind in the same class. For there is
no name common to all the objects that I mean, but, for all that,
these things are all in the same class by analogy. Compare what
Empedocles says: They are one and the same, hair and leaves and the
thick wings of birds and scales that grow on stout limbs.) The fumes