1163. Nitrogen is found in plants in very small proportions, and it is generally taken up by them combined with hydrogen, or, in other words, as ammonia. 'In fact,' as Liebig observes, 'science is at present ignorant of any compound of nitrogen except ammonia, which is capable of yielding nitrogen to the wild plants on all parts of the earth's surface.' (Liebig's Chemistry, p. 214.) Nitrogen is, however, occasionally supplied to plants in another form, that of nitric acid, and, with the exception of these compounds (nitric acid and ammonia), 'nitrogen exists only in the form of a gas which has been recognised as one of the constituent parts of atmospheric air.' (Ibid.) It was long supposed that plants possessed the power of assimilating the nitrogen contained in the atmosphere, though chemists were unable to explain how this was effected. The necessity for it, however, is now obviated by the discovery of ammonia in the air, from the remains of decayed animal matters being dispersed in the atmosphere.