A fundamental condition for the preparation of a good Vienna white is the employment of pure raw material, which must be free from ferric oxide or earthy impurities, and fully burned. An excellent material for this purpose is calcined mussel shells, which furnish a loose, and at the same time very pure, lime, and are very largely used for lime-burning in places such as Holland, where they are available in large quantities. Vienna white is not much used as a paint colour, owing to its powerful alkaline properties which have a destructive effect on many colours. It is, however, largely employed as a polishing agent, for which purpose it is powdered and is put on the market--mostly in bottles--as Vienna lime. Its very handsome white colour and low price render it particularly suitable for coarse painting, for example as a prime coating for painted interior walls. To guard against the danger of the painted decoration being destroyed by the alkaline nature of the white, it is advisable to coat the dried ground with alum solution, the alumina of which combines with the lime to form an insoluble compound to which organic colours adhere well. The sulphuric acid also enters into combination with the lime, the resulting gypsum having no effect on the paints subsequently applied. CHALK The name chalk is used for a number of commercial substances which differ considerably in both the mineralogical and chemical sense. French chalk, for instance, is a mineral belonging to the steatite group and, apart from its name, has nothing in common with true chalk, except the white colour, and even this differs altogether from that of chalk properly so called. It is therefore necessary, in the interests of proper nomenclature, to differentiate the various kinds of chalk, commencing with the mineral known by that name to the chemist and mineralogist. In chemical composition, true chalk is calcium carbonate, but of a fossil character, for if chalk dust be examined under a high-power microscope, it will be seen to consist of the shells of minute animals, and is therefore to be regarded as fossil. The organic matter of the animals has long disappeared, leaving the inorganic material, a very pure calcium carbonate, behind. [Illustration: FIG. 27.] Such progress has been made that the zoological status of the animals which inhabited the shells--many thousands of which are present in a lump of chalk--has been identified; and it is known that these animals were of marine type. Fig. 27 shows the appearance of the animal remains in Meudon chalk when highly magnified, the upper half being viewed by transmitted light and the lower by reflected light.