ed here.

~Methane~ (_marsh gas_) (CH_{4}). This is one of the most important of
these hydrocarbons, and constitutes about nine tenths of natural gas. As
its name suggests, it is formed in marshes by the decay of vegetable
matter under water, and bubbles of the gas are often seen to rise when
the dead leaves on the bottom of pools are stirred. It also collects in
mines, and, when mixed with air, is called _fire damp_ by the miners
because of its great inflammability, damp being an old name for a gas.
It is formed when organic matter, such as coal or wood, is heated in
closed vessels, and is therefore a principal constituent of coal gas.

~Preparation.~ Methane is prepared in the laboratory by heating sodium or
calcium acetate with soda-lime. Equal weights of fused sodium acetate
and soda-lime are thoroughly dried, then mixed and placed in a
good-sized, hard-glass test tube fitted with a one-holed stopper and
delivery tube. The mixture is gradually heated, and when the air has
been displaced from the tube the gas is collected in bottles by
displacement of water. Soda-lime is a mixture of sodium and calcium
hydroxides. Regarding it as sodium hydroxide alone, the equation is

NaC_{2}H_{3}O_{2} + NaOH = Na_{2}CO_{3} + CH_{4}.

~Properties.~ Methane is a colorless, odorless gas whose density is 0.55.
It is difficult to liquefy, boiling at -155 deg. under standard pressure,
and is almost insoluble in water. It burns with a pale blue flame,
liberating much heat, and when mixed with oxygen is very explosive.

~Davy's safety lamp.~ In 1815 Sir Humphry Davy invented a lamp for the use
of miners, to prevent the dreadful mine explosions then common, due to
methane mixed with air. The invention consisted in surrounding the upper
part of the common miner's lamp with a mantle of wire gauze and the
lower part with glass (Fig. 59). It has been seen that two gases will
not combine until raised to their kindling temperature, and if while
combining they are cooled below this point, the combination ceases. A
flame will not pass through a wire gauze because the metal, being a good
conductor of heat, takes away so much heat from the flame that the gases
are cooled below the kindling temperature. When a lamp so protected is
brought into an explosive mixture the gases inside the wire mantle burn
in a series of little explosions, giving warning to the miner that the
air is unsafe.

[Illustration: Fig. 59]

~Acetylene~ (C_{2}H_{2}). Th