GROUND-AIR IN ITS HYGIENIC RELATIONS. 281 



of tliut 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 

 rubhle-soil, gravel, or sand, which support the largest and heaviest 

 edifices. Here is a bottle which holds exactly one litre (ly^ 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 surfixce, 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 gi-avel 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 pi'oved 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 ai-e 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 

 poi'osity 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 filter- 

 ing 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 



