The pores of the ground are filled with water or air. The water may be reduced by drainage, but the ground-air will be increased by it. Sandy soils may contain air to the extent of 40 or even 50 per cent of their volume.

Ground-air is dangerous in more ways than one: it contains a relatively large proportion of carbonic acid, the proportion increasing with the distance from the surface. At Dresden, Flach found the ground-air to contain 3 per cent of the gas at a depth of 2 metres, and no less than 8 per cent at a depth of 6 metres; even 3 per cent is about one hundred times as much as that contained in normal atmospheric air. When we remember that air containing 2 per cent will extinguish a candle, the dangerous character of ground-air is easily understood.

A further and greater danger is that ground-air may contain other gases, such as ammonium sulphide and marsh-gas, due to the fermentation and decomposition of organic matters, and, in towns especially, may be charged with sewer-air and with coal-gas from leaking mains. The explosive character of air mixed with coal-gas is not its only danger; it may produce headache, nausea, and more distressing symptoms, while its presence may be unsuspected, as the gas may be deodorized by passing through the soil.

And finally, ground-air may contain the spores of pathogenic bacteria which have found a suitable nidus in the soil.

The movement of ground-air is a well-ascertained fact, and is influenced by wind and rain, by the rise and fall of ground-water, and by changes of tempera-ture and barometric pressure.

A warm house tends to draw the cool damp ground-air into it, especially when the external temperature is low. The explosions of coal-gas in houses which have do connections with the gas-mains are evidence in proof. Hence the necessity of covering the site of a house with a layer which shall be impervious alike to moisture and to gases, even under pressure.

"The site of every house or building shall be covered with a layer of good at least six inches thick, and smoothed on the upper surface "Such is the by-law in force in London. In many towns a layer of asphalt may be used instead of concrete. If good results are desired, both concrete and asphalt should be used, as concrete alone is not impervious, and asphalt is all the better for a firm and level bed. Examples of ground-layers are given in figs. 18, 21, 23, 25, 28, and also in Plates II. and III.

Concrete for ground-layers need not be as strong as that for foundations, but should be more solid; it must therefore contain more sand in proportion to the gravel or other broken material. Coke -breeze is not a good material for the aggregate; harder and more impervious material is better, such as gravel, broken stone, slag, hard brick, even broken flints. The aggregate, too, should be of smaller size; all should pass a screen with l -inch or 1 -ineh square meshes. The sand may be finer than that used for foundations. The proportions may be 1 cement + 2 sand + 4 aggregate, or 1 cement + 2 sand + 5 aggregate. The wet concrete should be well pounded, either with a light rammer or the back of a spade, and trowelled to a smooth surface. The last operation reduces considerably the perviousness of the concrete, and a further reduction may be effected by covering the layer with a coat, however thin, of neat cement, or even of lime-whiting.

Professor Tichborne found the relative porosity of certain mortars and asphalt to be as follows: - 1. Common lime-mortar (1 lime+ 2 sand), 100; 2. plaster of Paris, 75; 3. Roman cement, 25; 4. Portland cement, 10; 5. asphalt, 0. The advantage of asphalt ground-layers is evident from these figures. Portland cement, indeed, approaches asphalt, but Portland cement, it must be remembered, is not concrete: admixture with sand and other material increases its permeability, however carefully the admixture may be made.

Asphalt is a generic name, comprising many varieties of material, good, bad, and indifferent. It may mean the natural bituminous limestone found at Pyrimout Seyssel, Limmer and Vorwohle, Val de Travers, and other places, or more commonly, a compound of any one of these with more bitumen and sand. It is also used to designate a number of compounds, of which coal-tar pitch is the chief ingredient, but which may contain oil, sand, quicklime, tar, and other materials. These may be called "artificial asphalts" to distinguish them from the "natural asphalts".

Natural asphalts are undoubtedly more durable and trustworthy than the others, but they are also more expensive. The artificial "British" asphalt can be laid at one-half the cost of natural asphalt. For good work, however, one of the natural asphalts should be used. The preparation and laying of the material require considerable care, and ought always to be done by skilled men. An iron caldron is heated and gradually filled with small pieces of "mastic" asphalt, and a small proportion of natural bitumen to act as a flux, the mixture being stirred at intervals. In three or four hours from lighting the fire the mixture will be ready for use. Great care must be taken that the concrete bed is thoroughly dry, as otherwise the moisture will form steam and burst through the asphalt. To ensure the dryness of the bed, it should be allowed to stand for two or three weeks (more if possible), and hot cinders should be spread on it before the asphalt is laid. After the removal of the cinders, the caldron man takes a quantity of the viscous asphalt from the caldron, in a pail or ladle, and empties it on the bed; here the spreader at once commences to work it with a wooden rubber, carefully compressing it to the specified thickness, which need not be more, for ordinary work, than half an inch. Wood laths of the same thickness as the proposed coat of asphalt, are placed on the concrete to serve as guides in spreading the layer. Particular care must be observed that the junction between each new spreading and the work already executed is carefully made. In very wet situations, two ⅜ inch or inch layers must be used instead of the single layer, if perfect watertightness be desired.

The great objection to artificial asphalts is that they become very soft in hot weather, and are therefore apt to "run", and even to allow pieces of grit or sand to sink through them. They may, however, be used with advantage in buildings for which the natural asphalts are too costly. An artificial asphalt is better than no asphalt at all. They are chiefly mixtures of pitch and creosote oil, boiled in a caldron, a variable quantity of fine hot dry sand being subsequently added. Too much oil renders the asphalt soft and liable to run in hot weather, while excess of sand renders it brittle - especially in cold weather, - and may lead cracks or even general perviousness. The mixtures are usually laid in a more liquid state than the natural asphalts. The so-called "British" asphalt is a well-known variety. Briggs's "Tenax" and White's "Hygeian Rock" building-compositions arc better materials of similar nature, but supplied in powder ready for the caldron.

The various kinds of floor-surfaces suitable for basements will be described hereafter.