184 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
nitrogenous fertilisers, we have to consider also the fact that the 
demand in the future for these fertilisers will steadily increase. 
Public attention was first directed to the possibility 
of a “nitrogen famine” by Sir William Crookes in his 
Presidential Address to the British Association at Bristol 
in 1898. Not only did he point out the danger, but he also 
pointed out what he considered to be the remedy—the utilisation 
of the nitrogen of the air by causing it to combine with oxygen 
under the influence of electricity. At first sight this seems a 
very obvious way out of the difficulty, but the peculiar 
properties of free nitrogen makes the problem of its utilisation 
by no means such a simple matter. The element nitrogen, as it 
exists in the air, is a very inert substance, that is, it is only with 
the greatest difficulty that we can get it to combine with other 
elements to form the compounds of nitrogen, which alone are 
valuable from the fertilising point of view. 
The address of Sir William Crookes directed fresh public 
interest to the problem, which had long been a favourite one 
with chemists, and since 1898 reports have appeared from time 
to time announcing not only new methods of fixing the nitrogen 
of the air, but also that the methods have been a commercial 
success. With the possible exception of the methods to be 
considered in this article, these hopes have unfortunately 
not been realised, and we are still hardly in a position to 
say that the problem has been satisfactorily solved. Sir 
William Crookes pointed out that the essential condition for 
success was an abundant supply of electrical power at a 
sufficiently cheap rate, to be obtained probably by the utilisation 
of the water-power at present running to waste in many parts 
of the world. 
About 1785 the famous English chemist Cavendish showed 
that if you pass electric sparks through air, combination 
takes place between the oxygen and nitrogen, with the 
ultimate formation, in the presence of water, of small 
quantities of nitric acid. The same effect is seen in nature 
during a thunderstorm, the rain-water after a thunderstorm 
always containing more nitrates than the average amount 
present. Unfortunately, the amount of nitric acid formed by 
passing electric sparks through air is very small, and the cost of 
the electric energy required has hitherto prevented any success- 
ful application of the method on an industrial scale. It is 
