June 24, 1915] 
NATURE 
453 

been known that small quantities of ammonia may 
be formed by the action of high temperatures, say 
by the passage of electric sparks, on a mixture of 
hydrogen and nitrogen. But the reaction is 
necessarily incomplete, since it belongs to the class 
known as reversible, and in ordinary circum- 
stances the yield of ammonia is wholly incom- 
mensurate with its cost. But it was found by 
Haber that when a mixture of 1 part of nitrogen 
and 3 parts of hydrogen, under a pressure of 
175 atmospheres, is heated to about 550° in pre- 
sence of a catalyst, about 8 per cent. by volume 
of ammonia is formed, which may be isolated by 
passing the products through a _ refrigerating 
apparatus, the uncombined gases being returned 
to the compression chamber. 
The catalyst first used by Haber was osmium, 
‘a comparatively rare metal belonging to the 
platinum group. Later, finely powdered uranium 
was employed. Much experimental work has been 
spent in the effort to find other and cheaper cata- 
lysts, im studying the influence of temperature and 
pressure upon the yields, and in overcoming the 
technical difficulties inseparable from the construc- 
tion of apparatus of large size capable of with- 
standing such high pressures as, say, a couple of 
hundred atmospheres. 
The nitrogen is obtained from the air by the 
use of a Hampson or Linde liquefying apparatus, 
and subsequent fractionation on Claude’s system ; 
the hydrogen is made by passing steam over red- 
hot iron or heated coke. The ammonia is con- 
verted into nitric acid by oxidation under the in- 
fluence of a catalyst. The same principle is 
adopted in the method of Ostwald, by which 
ammonia, obtained from ‘‘nitrolim,” or, as it is 
called in Germany, “ Stickstoffkalk,” by a process 
to be described later, is mixed with air and passed 
through iron tubes into a chamber containing the 
contact-agent. The resulting products are led to 
a condensing plant, whereby, by suitable arrange- 
ments, which cannot be here described but are 
well known, it is claimed that from 85 to 90 per 
cent. of the theoretical yield of nitric acid can 
be obtained of a strength and purity suitable for 
the manufacture of explosives. The Ostwald pro- 
cess has been worked for some time at Gerthe, 
near Bochum, where it is said to have produced 
upwards of 1800 tons of nitric acid annually; but 
the experience of other countries where it has been 
in operation has been far less favourable, and it 
is doubtful whether a single Ostwald plant is now 
in use outside Germany. 
Up to the present time, the most successful of 
the factories which have been established for the 
utilisation of atmospheric nitrogen would appear 
to be that of the North-Western Cyanamide Com- 
pany, at Odda, on the Hardanger Fiord, Norway. 
This concern, which is largely financed by British 
capital, is operated by electrical energy furnished 
by a water supply capable of producing 80,000 
horse-power. This factory and the associated 
Alby works together produce calcium carbide, 
and “nitrolim,” a mixture of calcium cyanamide 
and carbon. Pure nitrogen is obtained from the 
NO. 2382, VOL. 95| 

air by a Linde plant driven by a 200 horse-power 
electric motor, and capable of producing 13,000 
cubic feet of nitrogen per hour. This gas is 
caused to react on calcium carbide (made by 
fusing lime with Welsh anthracite in electric 
furnaces) in electric retorts heated to a tempera- 
ture of S8oo°. “Nitrolim,” by treatment with 
superheated steam, yields calcium carbonate and 
ammonia, which latter substance can be converted 
into nitric acid by combustion, as already stated. 
The methods for the direct combination of 
nitrogen and oxygen to form nitric acid depend 
upon a reaction first pointed out independently 
by Priestley and Cavendish upwards of 130 years 
ago, and further elaborated, towards the close of 
the last century, by Sir William Crookes and 
Lord Rayleigh, who established the theoretical 
principles upon which the reaction proceeds. 
They showed that under the influence of a high 
temperature, produced by electric sparking, or by 
the passage of a strong induction current, oxides 
of nitrogen, and ultimately nitric acid, were 
formed in notable quantity. Indeed, it was in ‘ 
the course of the experiments which served to 
establish the composition of water that Cavendish 
incidentally discovered the true nature of nitric 
acid. But, as the history of science so frequently 
reveals, although the fundamental discovery was 
made by English observers, it was left to foreign 
technologists to turn it to practical account. This 
was first accomplished by Birkeland and Eyde in 
1903, who established a factory at Notodden, in 
Norway. In their process, air is driven by a 
Roots blower through a powerful arc flame, 
operating in a magnetic field, in a specially con- 
structed furnace. At the high temperature of the 
flame (3000°-3500°) about 1 per cent. of nitric 
oxide is formed, equal to about 30 milligrams of 
nitric acid per litre. The actual volume of air 
operated upon in each furnace is nearly 800,000 
litres per minute, and in all about three dozen 
furnaces are in use. The nitric oxide thus pro- 
duced is rapidly cooled, when it combines with a 
further amount of oxygen in the escaping gases 
to form nitric peroxide, which by treatment with 
water in absorption towers is changed into dilute 
nitric acid, to be subsequently concentrated or 
converted into nitrates. 
Various modifications in the mode of pro- 
ducing the arc flame, either with or without a 
magnetic field, have been introduced by German 
and Russian engineers, and different methods of 
absorption and concentration of the acid have been 
suggested, but the essential principles of the pro- 
cesses are practically identical in all of which pub- 
lished accounts are at present available. 
It will be seen from the foregoing statement 
that the Germans have by no means an exclusive 
monopoly in the production of synthetic nitric 
acid, and there is no reason to believe that the 
modifications they have been able to make in pre- 
existing processes not of their own invention have 
placed them in an independent or greatly superior 
position. It must be remembered that they are 
at present driven to work under abnormal and 
