of that air and its relations, we must know, in the first instance, its quantity in proportion to the different kinds of soil. Let us first take rubble-soil, gravel, or sand, which support the largest and heaviest edifices. Here is a bottle which holds exactly one litre (1710 pint) up to that mark on its neck. I have filled it slowly with gravel, shaking it all the while, so that the gravel settled completely. The gravel reaches up to the mark. This high cylinder contains just one litre of water, and is graduated into one hundred parts. Now I pour the water into the gravel, till I find it just coming up to its surface, and I see that of the water in the cylinder thirty-five parts have entered the gravel and driven out the air, which before had therefore taken up thirty-five per cent, of the whole mass. This is certainly a great quantity of air, and if we build a house on such a ground its weight rests, no doubt, on the gravel alone, and not on the air; but for all that, this ground, as far as it is dry, consists to the extent of one-third of air. In building on gravel, we build as well on air, just as we build on water when we build on piles driven into a swampy soil and cut off under the water. We know well that a house standing on piles stands with its foot in water, that this water is drawn up by the walls till beyond the water-mark, that the water of the ground has a good deal to do with the house; why should we, then, refuse to acknowledge that the foot of a house built on dry gravel, stands also on the air, and that the air in the ground is in intimate relation with the house?
What I have shown you in regard to gravel, can, in a similar way, be proved in regard to sand, clay, and even more solid stony and rocky soils.
Most kinds of sandstone are nearly as porous as loose sand. The rock of Malta has been proved by Leath Adams to suck up water on an average to one-third of its volume; consequently, when dry it must contain air to the same extent. One would not think that this was the case with the rugged cliffs and shores of that remarkable island, which look as if they were built up from the granite of the Swiss Alps. Most buildings in Malta are built with this Maltese rock, which is much used also throughout Italy. It is not less porous than the Berlin sands; their penetrability for air and water is the same, but the grains of the Maltese rock are connected by some solid medium, while the grains of the sand are loose. In respect to their porosity they stand relatively as frozen and not frozen soil.
Many ships of the English navy have filters made of a certain kind of Maltese rock. I have tested one, and I have found that the filtering basin swallows up forty-seven per cent, of its whole contents, when used for the first time.
A soil whose pores are filled partly by air and partly by water is called damp. It can take up more water till all its pores are filled with it, when all passage of air is stopped, just as we have seen
