1212. By means of pulverisation a portion of atmospheric air is buried in the soil. This air, so confined, is decomposed by the moisture retained in the earthy matters. Ammonia is formed by the union of the hydrogen of the water with the nitrogen of the atmosphere; and nitre, by the union of oxygen and nitrogen; carbonic acid gas, and carburetted hydrogen are also obtained from the air. Heat is given out during these processes, and 'hence,' as Dr. Darwin remarks (Phytoloqia, sect. xii. 1.), 'the great propriety of cropping lands immediately after they have been comminuted and turned over; and this the more especially, if manure has been added at the same time, as the process of fermentation will go on faster when the soil is loose, and the interstices filled with air, than afterwards, when it becomes compressed with its own gravity, the relaxing influence of rains, and the repletion of the partial vacuums formed by the decomposition of the enclosed air. The advantage of the heat thus obtained in exciting vegetation, whether in a seed or root, especially in spring, when the soil is cold, must be very considerable.'