Porosity may thus be regarded as the opposite of vitrification; porous goods being relatively light and soft whilst vitrified ones are dense and hard. For some purposes, porosity is an important characteristic: for example, building bricks which are moderately porous are preferable to those which are vitrified. The manufacture of porous blocks for the construction of light, sound-proof partitions, etc. has increased rapidly of late. They are made by adding sawdust or other combustible material to the clay. The added substances burn out on firing the goods in a kiln. Clays which are porous can be dried more readily and with less risk of cracking than those which are more dense. For this reason, some clayworkers mix non-plastic material such as sand or burned clay with their raw material. The _impermeability_ of plastic clay to water is a characteristic which is important for many purposes. The _absorptive power_ of clays is closely related to their porosity so far as pure water is concerned, but if the water contains certain salts in solution a selective absorption occurs, the bases being retained by the clay in such a manner that they cannot be removed by washing. The selective action is known as _adsorption_ and is most noticeable in highly plastic clays. Bourry (10) has shown that the slightly plastic china clays only exercise a small power of adsorbing calcium carbonate from solution, but highly plastic clays may adsorb 20 per cent. of it. The alkaline chlorides and sulphates do not appear to be adsorbed in this manner, but the carbonates are readily removed from solution. All calcium and magnesium compounds appear to be adsorbed, though in variable quantities, the reaction being complicated when several soluble salts are present. Ries (6) has found that gallo-tannic acid is adsorbed readily and increases the plasticity of clay. Ashley (11) has endeavoured to measure the plasticity of clays by determining their adsorption capacity for various aniline dyes, but his untimely decease prevented the investigation being completed. There is reason to suppose that the relation between adsorption and plasticity is extremely close in many clays and that the former may, to an important extent, be used as a measure of the latter. In some clays, however, this relationship does not exist. Sand and burned clay only show faint adsorption phenomena; felspar shows them to a slight and almost negligible extent and most of the other non-plastic ingredients of clays are non-adsorptive. Selective adsorption being an important characteristic of colloidal substances, the possession of this power by plastic clays supports the claim that plasticity is due, at least in part, to the presence of colloids.