Magnesia, which is present, for example, in dolomitic limestone, cannot be detected by the colour, either before or after burning, this oxide being itself perfectly white; but its presence is a drawback because if in large quantity it makes the lime very difficult to quench, and such lime is never of a fatty character. Organic matter betrays itself by the colour, the lime being dark tinted, varying from grey to black. Black limestones usually contain carbon in an extremely fine state of division, and are quite useless to the colour-maker owing to the impossibility of completely burning off this contained carbon, which always imparts a greyish tinge to the burnt lime. The behaviour of limestones in this respect varies, however, considerably, and can only be ascertained with certainty by a trial burning. Many that are rather dark in colour will, nevertheless, burn perfectly white, whereas others, much lighter in shade, always give a product that is not quite pure in tone. This divergent behaviour seems to have some connection with the chemical composition of the organic matter in question. If it consists of coal, or substances analogous thereto, no really pure white lime can be obtained from a light grey limestone, it being impossible to burn off the finely divided carbon completely. In addition to making a trial burning with a fairly large sample of material, the behaviour of a limestone towards hydrochloric acid will afford some information as to the nature of the grey colouring matter. If the limestone dissolves completely when suffused with the acid, the indications are favourable for its usefulness to the colour-maker. If, on the contrary, a black residue is left, the coloration is due to finely divided carbon, and there is then little prospect of the material furnishing a suitable product. In any event, a trial burning is the most reliable guide. In addition to carbon, the presence of any large proportion of ferric or ferrous oxide is objectionable, since, in either case, the product will be tinged red with ferric oxide, into which the ferrous oxide is transformed at calcination temperature. In addition to comparing the colour of the product with a standard sample, the suitability of a burnt lime for colour-making can be tested by quenching. If a lump about the size of the fist be placed in a large porcelain basin and suffused with a small quantity of water, preferably poured in a thin stream, the lime, if properly burned, will continue to absorb the water for a considerable time, like a sponge, and will very soon give evidence of a brisk reaction by increasing in bulk and generating such an amount of heat as to cause the immediate evaporation of a few drops of water allowed to fall on the surface of the mass. Finally, the entire lump will crumble down to a very delicate, voluminous powder, consisting of slaked lime (calcium hydroxide). This chemical reaction is expressed by the equation:--