}O.

The oxide of nitrogen so formed is called nitrous oxide or laughing gas.
It is a colorless gas having a slight odor. It is somewhat soluble in
water, and in solution has a slightly sweetish taste. It is easily
converted into a liquid and can be purchased in this form. When inhaled
it produces a kind of hysteria (hence the name "laughing gas"), and even
unconsciousness and insensibility to pain if taken in large amounts. It
has long been used as an anaesthetic for minor surgical operations, such
as those of dentistry, but owing to its unpleasant after effects it is
not so much in use now as formerly.

Chemically, nitrous oxide is remarkable for the fact that it is a very
energetic oxidizing agent. Substances such as carbon, sulphur, iron, and
phosphorus burn in it almost as brilliantly as in oxygen, forming oxides
and setting free nitrogen. Evidently the oxygen in nitrous oxide cannot
be held in very firm combination by the nitrogen.

[Illustration Fig. 39]

~Nitric oxide~ (NO). We have seen that when nitric acid acts upon metals,
such as copper, the reaction represented by the following equation takes
place:

3Cu + 8HNO_{3} = 3Cu(NO_{3})_{3} + 2NO + 4H_{2}O.

Nitric oxide is most conveniently prepared in this way. The metal is
placed in the flask A (Fig. 39) and the acid added slowly through the
funnel tube B. The gas escapes through C and is collected over
water.

Pure nitric oxide is a colorless gas, slightly heavier than air, and is
practically insoluble in water. It is a difficult gas to liquefy. Unlike
nitrous oxide, nitric oxide does not part with its oxygen easily, and
burning substances introduced into this gas are usually extinguished. A
few substances like phosphorus, which have a very strong affinity for
oxygen and which are burning energetically in the air, will continue to
burn in an atmosphere of nitric oxide. In this case the nitric oxide
loses all of its oxygen and the nitrogen is set free as gas.

~Action of nitric oxide with oxygen.~ When nitric oxide comes into contact
with oxygen or with the air, it at once combines with the oxygen even at
ordinary temperatures, forming a reddish-yellow gas of the formula
NO_{2}, which is called nitrogen peroxide. This action is not energetic
enough to produce a flame, though considerable heat is set free.

~Nitrogen peroxide~ (NO_{2}). This gas, as we have just seen, is formed by
allowing nitric oxide to come into contact with oxygen. It can also