Much of the rain absorbed by the surface-soil is, if the humidity of the atmosphere be considerably below saturation, returned to the air by evaporation, as is that which rises to the surface by capillary attraction. But heavy and repeated rainfalls force the water already in the soil progressively downwards, until it meets with a more or less impervious stratum of rock or clay, where it tends to accumulate, and to form a sheet of water filling the interstices between the particles of the soil. This "ground-water", as it is called, moves in the direction of the incline of the solid bed on which it rests, and with a velocity proportioned to the gradient and to the looseness of the soil in which it is held. If owing to denudation it come to the surface, a spring is the result. More often it makes its way to a river, the volume of which it helps to swell; but in sandy soils the reverse sometimes occurs, and the river by percolation through its bed serves to feed the ground-water. Such, indeed, is the sole source of the water obtained from wells in the valley of the Nile, where the rainfall is insignificant, and, owing to the extreme dryness of the air, evaporates almost immediately. Ground-water is everywhere liable to pollution by percolation from foul ponds or cess-pools, since the water from these enters the soil at a depth to which the bacteria, whose function it is to convert organic into inorganic matter, do not penetrate

When the upper layers of the soil are not saturated with water the interstices are occupied by air, largely composed of carbonic acid, and containing also other gases, the products of the decomposition of organic matter. With the rise and fall of the ground-water, and with variations in barometric pressure, the ground-air is forced out or atmospheric air drawn in. In order to prevent the rise of the ground-air and moisture into the dwelling - a danger to be especially feared in winter when the air of the house is warmer than the, ground-air, - it is necessary to cover the site of the house with an impervious layer of concrete or asphalt.

Pettenkofer, the great authority on all that concerns the movements of water and air in the soil, is of opinion that the carbonic acid in both is due to the action of bacteria, analogous to those of nitrification, on the organic matter in the soil, and that the fatal results of incautiously entering vaults in cemeteries have been caused, not by mephitic exhalations, but, precisely as in like descents into wells, by the accumulation of carbonic acid gas from the soil. But whatever other gases the ground-air of the country - that is to say, under natural conditions - may contain, there is no doubt as to the impurity of that in the town. Coal-gas is always present from the yielding of the joints in the gas-mains under the vibration of the traffic in the streets. Cases of poisoning, with symptoms not unlike those of typhoid, are often observed on the Continent and in America, where the winter is more severe than here, and especially in underground rooms, though there may be no gas laid on to the house. Pettenkofer has experimentally shown that it is not, as is commonly supposed, that the frozen surface-soil will not permit the gas to escape into the atmosphere, but that the ground-air is drawn, even for some hundreds of feet, in the direction of buildings the interiors of which are at temperatures much above that of the earth and outer air, and this, too, irrespective of the actual temperature or time of year. Coal-gas may produce serious consequences without being suspected, for the poisonous constituents are inodorous, the characteristic smell of gas being due to tarry and oily matters which the soil, until it becomes saturated with them, may effectually arrest

The temperature of the surface of the earth on a cloudy day follows the changes in that of the atmosphere, but when exposed to the direct rays of the sun, the soil absorbs heat and acquires a higher temperature. Dark, coarse, dry soils absorb most, and may attain a temperature of 50° C. (122° F.) or more; light-coloured, fine, and moist soils absorb least heat, and its emission by radiation is proportioned to the absorption, though, in consequence of evaporation, a soil cools more rapidly when damp than when dry Dry soils of any kind are thus rightly deemed warmer than those which are constantly damp.

"Made earth" is the euphemistic expression current among builders for the rubbish, ashes, dust, and "slop" with which they find it profitable to fill natural or other hollows of a building-site. Being, like the contents of dust-bins and the road-sweepings, of which it chiefly consists, largely composed of animal and vegetable refuse, it is liable to putrefy and ferment for years, with the gravest consequences to the health of the occupants of the houses as well as to the stability of the buildings. It is most to be feared on sites ostentatiously adver-tised as fine deep gravel or sand, since it pays the builder well to excavate and sell these materials, and to substitute rubbish which costs him nothing, and for taking which, indeed, in some cases, payment is made to him.

The influence of soil on health is undoubtedly modified to a great extent by drainage and building. No one now associates agues with the fashionable London quarter of Belgravia, yet at one time the marshes of Ebury were scarcely habitable.

Much has been written of late on the distribution of diseases in the British Isles, and many unintelligible conclusions as to the connection between the geological formation and the prevalence of certain diseases have been drawn from mere statistics, the vera causa being lost sight of meanwhile. That malarious disease - the so-called remittent, intermittent, or marsh fevers, and agues - are rooted to the soil in certain localities, no one doubts; nor that, apart from the bacterial theories of its causation, the "malaria requires for its development a soil containing organic matter in excess of the needs of the vegetation, together with a certain degree of temperature and of humidity, all three factors being essential." Unknown in arctic regions, it appears in such as Finland only in unusually warm summers; further south it prevails constantly in summer or autumn; and in hot climates it is always present except during prolonged drought Its absence from places at considerable elevations is only accidental, for white Mexico, Armenia, and elsewhere, the above-mentioned conditions are found, it may be met with at many thousand feet above the level of the sea. It is most prevalent while the ground-water is subsiding, but complete submergence or desiccation of the soil arrests it. Drainage of the subsoil, and cultivation, serve to reduce or even to abolish it, provided the crops are not, like rice and sugarcane, such as require irrigation. In climates, however, like those of West and Equatorial Africa, and in the deltas of great tropical rivers, the task of eradicating the disease is obviously impossible. Ague still holds its ground in the Fen