will not decay for ages. In the dry
climate of Egypt wooden mummy cases have been preserved for more than
three thousand years. On the other hand, dry wood burns much quicker
than green wood; it is not easy to set the latter on fire. Why this
difference, if decay and burning are similar processes? The decay of the
green wood is due to the fact that the presence of moisture causes
certain changes in portions of the wood, which enable the oxygen to
attack it at a low temperature; and the slow combustion, once started,
is self-sustaining. But in ordinary burning the temperature must be
raised to a certain point before the oxidation can begin, and this point
can not be reached until the moisture is evaporated, which uses up a
good deal of heat.

This process of decay is continually going on in our bodies; but during
life the matter which is burned up is being constantly renewed from the
food we eat. The body is not only decaying, as dead animal matter
decays, but it is also wearing out. With every motion a part of the
muscles is actually consumed, and must be replaced by fresh material.
The heat of the body is likewise due to combustion, and must be kept up
by proper fuel, like the fires in our stoves and furnaces. The products
of all this burning are carbonic acid and water, which pass out of the
body through the lungs.

The rusting of metals is a slow combustion, and scientific men have
proved that, like decay, it develops heat. Iron can be easily burned in
pure oxygen, with the production of intense light and heat. Zinc and
some other metals can be burned in the air if heated very hot, and most
metals are rapidly consumed in the flame of the oxyhydrogen blow-pipe.
Indeed, every form of matter known to us can be burned, unless it has
already been burned. All substances belong to one of these two
classes--those that will burn, or unite with oxygen; and those that have
been burned, or are products of oxidation. Water belongs to the latter
class, and so do nearly all the rocks and solid matter of the earth.

Slow burning sometimes becomes rapid, and then we have what is called
_spontaneous combustion_. When cotton or tow which has become soaked
with oil is laid aside in heaps, the oxygen of the air begins to unite
with it; but the heat developed causes the oxidation to go on faster and
faster, until in some cases the mass bursts into a flame. The same thing
sometimes takes place in moist hay, the moisture starting the proc