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
nitrogen at 194° C. The oxygen left is useful for other purposes. The
hydrogen needed is extracted by a similar process of fractional
distillation from "water-gas," the blue-flame burning gas used for
heating. Then the nitrogen and hydrogen, mixed in the proportion of one
to three, as shown in the reaction given above, are compressed to two
hundred atmospheres, heated to 1300° F. and passed over the finely
divided uranium. The stream of gas that comes out contains about four
per cent. of ammonia, which is condensed to a liquid by cooling and the
uncombined hydrogen and nitrogen passed again through the apparatus.

The ammonia can be employed in refrigeration and other ways but if it is
desired to get the nitrogen into the form of nitric acid it has to be