┌──────────────────────────────────────────┬────────────┬─────────────┐ │ │PERMEABILITY│ EMANATIONS │ │ │ OF WATER. │ INTO AIR. │ ├──────────────────────────────────────────┼────────────┼─────────────┤ │1. _Primitive rocks, clay slate, millstone│ │ │ │ grit_ │ Slight. │ None. │ │2. _Gravel and loose sands, with permeable│ │ │ │ subsoils_ │ Great. │ Slight. │ │3. _Sandstones_ │ Variable. │ Slight. │ │4. _Limestones_ │ Moderate. │ ── │ │5. _Sands with impermeable subsoil │Arrested by │Considerable.│ │ │ subsoils. │ │ │6. _Clays, marls, alluvial soils_ │ Slight. │Considerable.│ │7. _Marshes, when not peaty_ │ Slight. │Considerable.│ └──────────────────────────────────────────┴────────────┴─────────────┘ The general geological conditions have an important bearing on the choice of a site for a house in so far as they affect the local climate, and the difficulty of keeping the house warm and dry. Pettenkofer expressed this in his dictum, that we take holiday for change of soil, rather than for change of air. The character of a soil has an important influence on humidity, radiation, evaporation, and in fact most of the factors going to make up “climate.” The immediate local surroundings of a house (page 201) have an even greater influence on its salubrity than the underlying geological formation. The soil consists of mineral and organic matters. On the amount and character of the animal and vegetable matters (along with the condition of moisture and aeration), the healthiness of a given soil depends. The presence of vegetable matter, subject to alternate wettings and dryings, and to heat, has until recently been regarded as the condition on which malaria depends; but it is now known that malarial places owe their character to their being favourable to the growth of the larvæ of certain mosquitoes (page 307); and that drainage of the soil cures malaria by removing the ponds in which these develop. The two chief agencies at work to rid the soil of organic impurities, are nitrification and the influence of growing plants. The organic matters become oxidised into ammonia, nitrites, and nitrates, and these are eagerly assimilated by vegetation.