y become white hot or burst
into a flame.

~Combustion; kindling temperature.~ When oxidation takes place so rapidly
that the heat generated is sufficient to cause the substance to glow or
burst into a flame the process is called _combustion_. In order that any
substance may undergo combustion, it is necessary that it should be
heated to a certain temperature, known as the _kindling temperature._
This temperature varies widely for different bodies, but is always
definite for the same body. Thus the kindling temperature of phosphorus
is far lower than that of iron, but is definite for each. When any
portion of a substance is heated until it begins to burn the combustion
will continue without the further application of heat, provided the heat
generated by the process is sufficient to bring other parts of the
substance to the kindling temperature. On the other hand, if the heat
generated is not sufficient to maintain the kindling temperature,
combustion ceases.

~Oxides.~ The compounds formed by the oxidation of any element are called
_oxides_. Thus in the combustion of sulphur, phosphorus, and iron, the
compounds formed are called respectively oxide of sulphur, oxide of
phosphorus, and oxide of iron. In general, then, _an oxide is a compound
of oxygen with another element_. A great many substances of this class
are known; in fact, the oxides of all the common elements have been
prepared, with the exception of those of fluorine and bromine. Some of
these are familiar compounds. Water, for example, is an oxide of
hydrogen, and lime an oxide of the metal calcium.

~Products of combustion.~ The particular oxides formed by the combustion
of any substance are called _products of combustion_ of that substance.
Thus oxide of sulphur is the product of the combustion of sulphur; oxide
of iron is the product of the combustion of iron. It is evident that the
products of the combustion of any substance must weigh more than the
original substance, the increase in weight corresponding to the amount
of oxygen taken up in the act of combustion. For example, when iron
burns the oxide of iron formed weighs more than the original iron.

In some cases the products of combustion are invisible gases, so that
the substance undergoing combustion is apparently destroyed. Thus, when
a candle burns it is consumed, and so far as the eye can judge nothing
is formed during combustion. That invisible gases are formed, however,
and that the weight of