ion with the manufacture of sodium hydroxide.

~Physical properties.~ Chlorine is a greenish-yellow gas, which has a
peculiar suffocating odor and produces a very violent effect upon the
throat and lungs. Even when inhaled in small quantities it often
produces all the symptoms of a hard cold, and in larger quantities may
have serious and even fatal action. It is quite heavy (density = 2.45)
and can therefore be collected by displacement of air. One volume of
water under ordinary conditions dissolves about three volumes of
chlorine. The gas is readily liquefied, a pressure of six atmospheres
serving to liquefy it at 0 deg.. It forms a yellowish liquid which
solidifies at -102 deg..

~Chemical properties.~ At ordinary temperatures chlorine is far more
active chemically than any of the elements we have so far considered,
with the exception of fluorine; indeed, it is one of the most active of
all elements.

1. _Action on metals._ A great many metals combine directly with
chlorine, especially when hot. A strip of copper foil heated in a burner
flame and then dropped into chlorine burns with incandescence. Sodium
burns brilliantly when heated strongly in slightly moist chlorine. Gold
and silver are quickly tarnished by the gas.

2. _Action on non-metals._ Chlorine has likewise a strong affinity for
many of the non-metals. Thus phosphorus burns in a current of the gas,
while antimony and arsenic in the form of a fine powder at once burst
into flame when dropped into jars of the gas. The products formed in all
cases where chlorine combines with another element are called
_chlorides_.

3. _Action on hydrogen._ Chlorine has a strong affinity for hydrogen,
uniting with it to form hydrochloric acid. A jet of hydrogen burning in
the air continues to burn when introduced into a jar of chlorine, giving
a somewhat luminous flame. A mixture of the two gases explodes violently
when a spark is passed through it or when it is exposed to bright
sunlight. In the latter case it is the light and not the heat which
starts the action.

4. _Action on substances containing hydrogen._ Not only will chlorine
combine directly with free hydrogen but it will often abstract the
element from its compounds. Thus, when chlorine is passed into a
solution containing hydrosulphuric acid, sulphur is precipitated and
Hydrochloric acid formed. The reaction is shown by the following
equation:

H_{2}S + 2Cl = 2HCl + S.

With ammonia the action is si