The treatment of the =internal wall-surface= of a room differs according to circumstances. =Lime-washing= is suitable only for stables and other outbuildings. It is made by the addition of water to quicklime, no size being added. It is an excellent germicide and insecticide. =Whitewashing= is quite different from limewashing. “Whiting,” _i.e._ finely-ground chalk, to which a certain proportion of size and alum had been added is mixed with water. The size and alum are added to prevent the whitewash from being rubbed off. =Distempering= is identical with whitewashing, except that pigments are added. It is distinguished from =painting in oils=, by the fact that the pigments are mixed with size, instead of with linseed-oil and turpentine. Painting in distemper is practically limited to plaster, which should first receive a coat of whitewash to diminish its porosity. Oil-paints are impervious, distemper is as absorbent as plaster or whitewash. Various =washable distempers=, as duresco, are made, which are more durable and non-absorbent. =Water-glass= consists of silicate of potash, which in the gelatinous form is soluble like size in hot-water, but when allowed to dry forms an impervious film. It can be used for protecting porous stone from the effects of weather; and renders internal surfaces of walls non-absorbent and washable. =Oil-painting= renders wall-surfaces impervious, and enables them to be easily washed. The importance of this in the event of any infectious disease occurring, is obvious. The question arises whether distempered or papered walls, which are porous, or painted walls, which are non-porous, are preferable from the standpoint of health. The difference between the two is seen during damp weather, when moisture condenses and runs down the latter and is invisible in the former. In practice in domestic dwellings the former are preferred; but although some advantage is thus secured in ventilation through the wall-substance, there is the serious disadvantage that particles of dirt accumulate and may seriously interfere with the purity of the air of a room. Hence the importance of rubbing down the internal surface of a room, whether distempered or papered, at intervals with bread crumb or dough (see page 332). This effectually removes all accumulations of dirt. A painted wall presents the enormous advantage that it can be frequently washed; while the loss of ventilation may be ignored, if windows and doors be properly utilised for this purpose. The presence of poisonous pigments in oil-paints is of importance to the workman, but not to the householder except during the painting, as paint, unlike distemper, does not rub off the wall. Lead is the chief poison present, as white lead (carbonate of lead). Various substitutes for lead paints have been introduced.