This substance is extremely valuable for the colouring of porcelain and glass; as it resists without change, the effects of the most intense heat. Hence also it is advantageously used for giving various shades of blue to enamels, and to glass manufactured in imitation of lapis lazuli, turquoise, sapphire, and various precious stones. So intense is the colour imparted by it that a single grain of zaffre will give a full blue tint to 240 grains of glass. _Smalt_ is a kind of glass, of dark blue colour, formed by melting zaffre with three parts of sand and one of potash; when this substance is ground to a coarse powder, it has the name of _strewing─smalt_, and is much used by sign painters, as an ornamental filling up of the vacant space betwixt the letters of signs. In Germany it is frequently employed instead of sand for the purpose of drying ink after writing. The same substance reduced to a perfectly fine or impalpable powder, is the article which is sold under the name of _powder─blue_, and which is not only used by laundresses and others in the getting up of linen, but also as the basis of several kinds of paint; and by the manufacturers of writing and printing papers, to give a blue tinge to those articles. A solution of the oxide of cobalt in spirit of salt (muriatic acid, 29) and afterwards diluted till nearly the whole of its colour disappears, forms one of the most beautiful _sympathetic inks_ with which we are acquainted. If a landscape be drawn with Indian ink, and, afterwards, the foliage be washed over with this solution, it will have no peculiar appearance; but, on holding the paper near the fire, the part representing the vegetation will gradually assume a green tint, which will subside on removing the paper into a cool situation. 248. _MANGANESE, in the state that we usually see it, is a black oxide of a metal which is of a silvery grey colour, of leafy or foliated texture, and somewhat more than six times as heavy as water._ Mines of manganese have long been worked in several parts of Great Britain, but particularly in the counties of Devon and Somerset. Near Exeter and in the Mendip Hills this mineral is found in great abundance. It is employed for various useful purposes. In the manufacture of the finer kinds of glass it is used in a double capacity, both as a colouring material and as a destroyer of colour. As a colouring ingredient, the imitators of several precious stones are indebted to it for the red and purple tints which they give to the oriental ruby, the balais ruby, and the amethyst.