1208. Another important use of the pulverisation of the soil, is to admit atmospheric air to the spongioles of the roots. If the roots are exposed entirely to the air, they become dry and withered, and lose their power of expansion and contraction; but though it is necessary to have the roots covered with soil, the particles of that soil should be as loose to admit the passage of air as it was before stated to be to admit finely divided particles of water. It must never be forgotten that a great portion of the carbonic acid gas taken up by the spongioles of plants, is obtained from the atmospheric air; and that if the roots of plants are covered with soil caked together so as to be impervious to air, the plants cannot obtain their proper supply of carbon, and consequently there will be a deficiency in their products. Fruit trees will not produce fruit, and timber trees will not produce durable timber, if the spongioles of their roots are entirely deprived of access to atmospheric air.