In the lower layers of the soil, down to 3 feet and 6 feet, micro-organisms are more protected against the injurious influences of the atmosphere, sunlight and drying up, but the want of oxygen, together with the greater difficulty of removing such products as carbonic acid, has an injurious influence. As the temperature in these layers is considerably more uniform, it may be inferred that the micro-organic activity is there of a more uniform kind, less influenced by sudden changes, probably also less intense, but without pronounced periods of rest. [Sidenote: Micro-organisms probably quickly perish in depths greater than 6 ft.] In depths greater than 6 feet micro-organisms probably perish very quickly owing to unfavourable conditions, and if found their presence must be explained by emigration from higher layers, not by actual growth at these depths. On sewage farms the micro-organic activity is without doubt greatly modified, and proceeds all the year round at a more uniform rate than on ordinary land, as the sewage always contains the necessary warmth and moisture so beneficial for it. 10. THE ABSORBING POWERS OF SOIL. [Sidenote: Absorbing powers due to surface attraction of the particles of the soil.] [Sidenote: The finer the pores the greater the absorption.] The absorbing powers of soil are due to the surface attraction of its particles or grains, and these, as has already been pointed out, will be all the greater the finer the pores are; they extend on the one hand to aqueous and other vapours and gases, and on the other to matters in solution. [Sidenote: 1 cub. yd. of coarse gravel may contain 50 sq. yds. of surface and 1 cub. yd. of fine sand 9200 sq. yds.] That the attractive force of the surface of the particles is pretty considerable will be at once apparent when it is stated that 1 cubic yard of coarse gravel may contain about 140,000 grains with a combined surface of 50 square yards, and 1 cubic yard of fine sand 40 million grains with a combined surface of 9200 square yards, which is a little under 2 acres. [Sidenote: Deodorising action of soil absorption of gases.] Concerning the absorption by soil of aqueous vapour and gases (apart from condensation through a fall in temperature), dry soil with fine pores acts most energetically. The almost instantaneous deodorisation of foul-smelling gases, such as are formed by decomposing fæcal matters (earth closet) or coal gas, through a thin layer of fine dry soil is well known, and is to be explained in this way. [Sidenote: Absorption of dissolved substances by soil.]