An important property of lime is its behaviour towards casein, the substance forming the curd of milk. With this body it combines to form a mass which sets hard and is highly resistant, viz. calcium caseate, and is formed when limewash is stirred up with milk or freshly precipitated casein. Weatherproof distempers for outside use are prepared in this manner. PEARL WHITE The preparation frequently met with in commerce under this name is nothing more than a burnt lime of great purity. It is prepared in the coastal districts by burning oyster shells, the resulting burnt lime being easily transformed into a fine powder, the pure white colour of which is due to the absence of iron. It is used in the same way as pure burnt lime, and is mainly of interest in seaside towns where oyster shells are often accumulated. It may be pointed out that the name pearl white is often applied also to pure white grades of white lead. VIENNA WHITE This colour is prepared from any kind of burnt lime that is sufficiently pure; that is, free from ferric oxide. The method of preparation is simple, requiring no special apparatus, and can therefore be carried out wherever suitable lime is available. Operations are commenced by carefully slaking well-burnt lime with water, a sufficient excess of which is added to produce a fairly thick pulp. To accelerate the absorption of carbon dioxide, the mass is exposed to the air in thin layers, by spreading it out on boards, so as to present a large surface to the air. As soon as the pulpy character has disappeared, the mass is detached from the boards, and is pressed and kneaded, with wooden paddles, into prismatic cakes which are left exposed to the air--being, of course, protected from the wet--until the absorption of carbon dioxide is complete--a condition that can be recognised by the earthy character of the product. The cakes are then dried, an operation entailing great care, since lightness is a sign of good quality, whereas a damp product is very heavy. In forming the cakes they must not be touched by the bare hands, because the lime is so caustic that it would soon destroy the skin. The foregoing method of manufacture is capable of many improvements, which can be introduced without adding much to the cost of production. If the lime is formed into large blocks, it will evidently take a long time for the mass to acquire, all through, the earthy character indicating combination with carbon dioxide. This drawback can be easily remedied by forming the mass into small cakes, which will become ripe, owing to their larger surface, much sooner than the bigger blocks.