to us particles of every variety of shape and structure, of which a certain part is evidently of vegetable origin. In these figures I have given a very rude representation of these particles; and I must beg you particularly to remember that they are not meant to represent by any means accurately what the microscope exhibits, but are only designed to serve as a plan by which to illustrate the mechanical properties of the soil. On referring to Fig. 91, we perceive that there are two distinct classes of pores; first, the large ones, which exist _between_ the particles of soil, and second, the very minute ones, which occur in the particles themselves; and you will at the same time notice, that whereas all the larger pores--those between the particles of soil--communicate most freely with each other, so that they form canals, the small pores, however freely they may communicate with one another in the interior of the particle in which they occur, have no direct connection with the pores of the surrounding particles. Let us now, therefore, trace the effect of this arrangement. In Fig. 91, we perceive that these canals and pores are all empty, the soil being _perfectly dry_; and the canals communicating freely at the surface with the surrounding atmosphere, the whole will of course be filled with air. If in this condition, a seed be placed in the soil, as at _a_, you at once perceive that it is freely supplied with air, _but there is no moisture_; therefore, when soil is _perfectly dry_, a seed cannot grow. [Illustration: Fig. 91.] [Illustration: Fig. 92.] "Let us turn our attention now to Fig. 92. Here we perceive that both the pores and canals are no longer represented white, but black, this color being used to indicate water; in this instance, therefore, water has taken the place of air, or, in other words, the soil is _very wet_. If we observe our seed _a_ now, we find it abundantly supplied with water, but _no air_. Here again, therefore, germination cannot take place. It may be well to state here, that this can never occur _exactly_ in nature, because water having the power of dissolving air to a certain extent, the seed _a_ in Fig. 92 is, in fact, supplied with a _certain_ amount of this necessary substance; and, owing to this, germination does take place, although by no means under such advantageous circumstances as it would were the soil in a better condition. [Illustration: Fig. 93.] [Illustration: Fig. 94.]