es left behind are only what can not be burned or united
with the oxygen. If we collect all the products of the burning, together
with the ashes, we find that they weigh more than the coal or wood, the
increase being exactly equal to the weight of the oxygen consumed. No
kind of matter can be destroyed by any power known to us; it may unite
with other matter, and take many new forms, but its weight can be
neither increased nor diminished. The amount of matter in the universe
is always the same.

Oxygen must be heated before it will unite with coal or wood. The air is
at all times in contact with them, but they will not burn unless they
are first kindled. The chemical process itself, when once started,
generally produces heat enough to raise more oxygen to the proper
temperature, and thus the combustion is kept up. The point to which the
oxygen must be heated varies much with different substances, as is well
shown in kindling a coal fire. The heat produced by rubbing a match on a
rough surface suffices to make the oxygen unite with the phosphorus on
the end of the match; the burning of this causes heat enough for the
union of the oxygen with the sulphur, and the burning of the sulphur
enough to set the wood of the match on fire. The shavings, the kindling
wood, and the charcoal are in turn ignited, and the burning charcoal
develops heat enough to enable the oxygen to combine with the hard coal.
Each step in the operation requires more heat than the preceding step.
This seems a very simple thing now, but the anthracite beds of
Pennsylvania long remained useless because no one had found out how to
kindle the fuel, and the discovery was at last made half by accident.

There are some forms of combustion which are very unlike ordinary
burning, and yet are essentially the same, being cases of union with
oxygen. The only difference is that the process goes on slowly instead
of rapidly. We know that vegetable and animal substances decay when
exposed to the air; and decay is a slow burning. The oxygen of the air
gradually combines with the substances, converting them into carbonic
acid and water, and leaving only a small remnant of matter as the ashes
of the lingering combustion. The _heat_ produced in this case is found
to be precisely the same as in ordinary burning, but it is set free so
gradually that it escapes our notice.

We know that green wood decays much sooner than dry wood. Indeed, if
wood is kept perfectly dry, it