these is greater than the weight of the candle
may be shown by the following experiment.

[Illustration: Fig. 7]

A lamp chimney is filled with sticks of the compound known as
sodium hydroxide (caustic soda), and suspended from the beam of
the balance, as shown in Fig. 7. A piece of candle is placed on
the balance pan so that the wick comes just below the chimney,
and the balance is brought to a level by adding weights to the
other pan. The candle is then lighted. The products formed pass
up through the chimney and are absorbed by the sodium
hydroxide. Although the candle burns away, the pan upon which
it rests slowly sinks, showing that the combustion is attended
by an increase in weight.

~Combustion in air and in oxygen.~ Combustion in air and in
oxygen differs only in rapidity, the products formed being
exactly the same. That the process should take place less
rapidly in the former is readily understood, for the air is
only about one fifth oxygen, the remaining four fifths being
inert gases. Not only is less oxygen available, but much of the
heat is absorbed in raising the temperature of the inert gases
surrounding the substance undergoing combustion, and the
temperature reached in the combustion is therefore less.

~Phlogiston theory of combustion.~ The French chemist Lavoisier
(1743-1794), who gave to oxygen its name was the first to show
that combustion is due to union with oxygen. Previous to his
time combustion was supposed to be due to the presence of a
substance or principle called _phlogiston_. One substance was
thought to be more combustible than another because it
contained more phlogiston. Coal, for example, was thought to be
very rich in phlogiston. The ashes left after combustion would
not burn because all the phlogiston had escaped. If the
phlogiston could be restored in any way, the substance would
then become combustible again. Although this view seems absurd
to us in the light of our present knowledge, it formerly had
general acceptance. The discovery of oxygen led Lavoisier to
investigate the subject, and through his experiments he arrived
at the true explanation of combustion. The discovery of oxygen
together with the part it plays in combustion is generally
regarded as the most important discovery in the history o