wer continuously in the fixation of nitrogen and the rest of the
world as much again. The Germans had invested largely in these foreign
oxidation plants, but shortly before the war they had sold out and
turned their attention to other processes not requiring so much
electrical energy, for their country is poorly provided with water
power. The Haber process, that they made most of, is based upon as
simple a reaction as that we have been considering, for it consists in
uniting two elemental gases to make a compound, but the elements in this
case are not nitrogen and oxygen, but nitrogen and hydrogen. This gives
ammonia instead of nitric acid, but ammonia is useful for its own
purposes and it can be converted into nitric acid if this is desired.
The reaction is:

NN + HH + HH + HH --> NHHH + NHHH
Nitrogen hydrogen ammonia

The animals go in two by two, but they come out four by four. Four
molecules of the mixed elements are turned into two molecules and so the
gas shrinks to half its volume. At the same time it acquires an
odor--familiar to us when we are curing a cold--that neither of the
original gases had. The agent that effects the transformation in this
case is not the electric spark--for this would tend to work the reaction
backwards--but uranium, a rare metal, which has the peculiar property of
helping along a reaction while seeming to take no part in it. Such a
substance is called a catalyst. The action of a catalyst is rather
mysterious and whenever we have a mystery we need an analogy. We may,
then, compare the catalyst to what is known as "a good mixer" in
society. You know the sort of man I mean. He may not be brilliant or
especially talkative, but somehow there is always "something doing" at a
picnic or house-party when he is along. The tactful hostess, the salon
leader, is a social catalyst. The trouble with catalysts, either human
or metallic, is that they are rare and that sometimes they get sulky and
won't work if the ingredients they are supposed to mix are unsuitable.

But the uranium, osmium, platinum or whatever metal is used as a
catalyzing agent is expensive and although it is not used up it is
easily "poisoned," as the chemists say, by impurities in the gases. The
nitrogen and the hydrogen for the Haber process must then be prepared
and purified before trying to combine them into ammonia. The nitrogen is
obtained by liquefying air by cold and pressure and then boiling off the
nit