equation for the
reaction.

6. Lead has a valence of 2 and iron of 3 in the compounds known
respectively as lead nitrate and ferric sulphate. (a) Write the
formulas for these two compounds. (b) When their solutions are mixed
the two metals exchange places; write the equation for the reaction.

CHAPTER XII

COMPOUNDS OF NITROGEN

~Occurrence.~ As has been stated in a former chapter, nitrogen constitutes
a large fraction of the atmosphere. The compounds of nitrogen, however,
cannot readily be obtained from this source, since at any ordinary
temperature nitrogen is able to combine directly with very few of the
elements.

In certain forms of combination nitrogen occurs in the soil from which
it is taken up by plants and built into complex substances composed
chiefly of carbon, hydrogen, oxygen, and nitrogen. Animals feeding on
these plants assimilate the nitrogenous matter, so that this element is
an essential constituent of both plants and animals.

~Decomposition of organic matter by bacteria.~ When living matter dies and
undergoes decay complicated chemical reactions take place, one result of
which is that the nitrogen of the organic matter is set free either as
the element nitrogen, or in the form of simple compounds, such as
ammonia (NH_{3}) or oxides of nitrogen. Experiment has shown that all
such processes of decay are due to the action of different kinds of
bacteria, each particular kind effecting a different change.

~Decomposition of organic matter by heat.~ When organic matter is strongly
heated decomposition into simpler substances takes place in much the
same way as in the case of bacterial decomposition. Coal is a complex
substance of vegetable origin, consisting largely of carbon, but also
containing hydrogen, oxygen, and nitrogen. When this is heated in a
closed vessel so that air is excluded, about one seventh of the nitrogen
is converted into ammonia, and this is the chief source from which
ammonia and its compounds are obtained.

COMPOUNDS OF NITROGEN WITH HYDROGEN

~Ammonia~ (NH_{3}). Several compounds consisting exclusively of nitrogen
and hydrogen are known, but only one, ammonia, need be considered here.

~Preparation of ammonia.~ Ammonia is prepared in the laboratory by a
different method from the one which is used commercially.

1. _Laboratory method._ In the laboratory ammonia is prepared from
ammonium chloride, a compound having the formula NH_{4}Cl, and obtained
in the man