The conclusions to be drawn from these considerations have been further confirmed by the direct experiments of different observers. Boussingault sowed peas, weighing 15.60 grains, in a soil composed of a mixture of sand and clay, which had been heated red-hot, and consequently contained no humus, and after 99 days' growth, during which they had been watered with distilled water, he found the crop to weigh 68.72 grains, so that there had been a fourfold increase. Similar experiments have been made by Prince Salm Horstmar, on oats and rape sown in a soil deprived of organic matter by ignition, in which they grew readily, and arrived at complete maturity. One oat straw attained a height of three feet, and bore 78 grains; another bore 47; and a third 28--in all 153. These when dried at 212 deg. weighed 46.302 grains, and the straw 45.6 grains. The most satisfactory experiments, however, are those of Weigman and Polstorf, these observers having found that it was possible to obtain a two-hundred-fold produce of barley in an entirely artificial soil, provided care was taken to give it the _physical_ characters of a fertile soil. They prepared a mixture of six parts of sand, two of chalk, one of white bole, and one of wood charcoal; to which was added a small quantity of felspar, previously fused with marble and some soluble salts, so as to imitate as closely as possible the inorganic parts of a soil, and in it they planted twelve barley pickles. The plants grew luxuriantly, reaching a height of three feet, and each bearing nine ears, containing 22 pickles. The grain of the twelve plants weighed 2040 grains.