The chief requirement exacted of a good quality chalk is a handsome white colour; and this depends entirely on the quality of the raw material, not on the method of preparation. It is known that a substance quite devoid of colour will furnish a perfectly white powder, because the colourless particles reflect the light in all directions without breaking it up into its constituent yellow, red and blue rays. Chalk, too, is in reality a colourless substance, and reflects light with greater uniformity in proportion as the fineness of the particles increases. Consequently, when one has a chalk that is not perfectly white, it can, nevertheless, be made to furnish a very handsome product by bestowing great care on grinding and levigation. Properly prepared chalk should be as fine as the finest flour. When the colour of the best grades of chalk are compared with what may be termed pure white--such as that of white lead, zinc white, permanent white--a skilled eye will always detect a greyish or yellowish tinge in the former, even if obtained from the whitest Carrara marble. The grey tinge is due to the presence of organic matter, which cannot be eliminated by any known means, but which can be shown to exist by the fact that when such chalk is heated to incandescence in the air for a short time, the resulting burnt lime is pure white, the organic matter having been burned off. A yellow tinge is caused by minute traces of ferric oxide, which--as also ferrous oxide--almost invariably accompanies calcium carbonate; and limestone free from determinable quantities of these oxides is of rare occurrence. Ferrous oxide does not reveal its presence in limestone unless in large proportion, its pale green colour being of low tinctorial power, whereas ferric oxide, which is a very strong colouring agent, can be more readily detected. To those who are engaged in the manufacture of white earth colours, however, it is quite immaterial whether a limestone or chalk contains ferrous oxide, because that oxide quickly changes into ferric oxide in the finely divided product, and a chalk which was originally pure white will become decidedly yellow in a short time. Fortunately, such a yellow-tinged product can be rendered perfectly white by simple means and at small cost, all that is necessary being to add a suitable quantity of a blue colouring matter. When this has been done, the chalk will seem pure white to even the most skilled eye. This result of adding a blue pigment is based on the well-known physical fact that certain kinds of coloured light produce white light when combined, the colours that give this effect being termed “complementary.” A pure blue is complementary to a yellow with a reddish cast--_e. g._ ferric oxide--and therefore a chalk that is tinged yellow by a small quantity of ferric oxide can be changed into a seemingly pure white substance by the addition of a blue pigment.