um hydroxide be substituted for the solution
of calcium hydroxide in testing for carbon dioxide?

15. What weight of sodium hydroxide is necessary to neutralize the
carbonic acid formed by the action of hydrochloric acid on 100 g. of
calcium carbonate?

16. What weight of calcium carbonate would be necessary to prepare
sufficient carbon dioxide to saturate 10 l. of water at 15 deg. and under
ordinary pressure?

17. On the supposition that calcium carbide costs 12 cents a kilogram,
what would be the cost of an amount sufficient to generate 100 l. of
acetylene measured at 20 deg. and 740 mm.?

18. How would the volume of a definite amount of carbon monoxide compare
with the volume of carbon dioxide formed by its combustion, the
measurements being made under the same conditions?

CHAPTER XVIII

FLAMES,--ILLUMINANTS

~Conditions necessary for flames.~ It has been seen that when two
substances unite chemically, with the production of light and heat, the
act of union is called combustion. When one of the substances undergoing
combustion remains solid at the temperature occasioned by the
combustion, light may be given off, but there is no flame. Thus iron
wire burning in oxygen throws off a shower of sparks and is brilliantly
incandescent, but no flame is seen. When, however, both of the
substances are gases or vapors at the temperature reached in the
combustion, the act of union is accompanied by a flame.

~Flames from burning liquids or solids.~ Many substances which are liquids
or solids at ordinary temperatures burn with a flame because the heat of
combustion vaporizes them slowly, and the flame is due to the union of
this vapor with the gas supporting the combustion.

~Supporter of combustion.~ That gas which surrounds the flame and
constitutes the atmosphere in which the combustion occurs is said to
support the combustion. The other gas which issues into this atmosphere
is said to be the combustible gas. Thus, in the ordinary combustion of
coal gas in the air the coal gas is said to be combustible, while the
air is regarded as the supporter of combustion. These terms are entirely
relative, however, for a jet of air issuing into an atmosphere of coal
gas will burn when ignited, the coal gas supporting the combustion.
Ordinarily, when we say that a gas is combustible we mean that it is
combustible in an atmosphere of air.

[Illustration: Fig. 62]

~Either gas may be the supporter of combustion.~ That