The raw materials are selected with great care to assure chemical purity, but whereas in most glasses the only impurities to be dreaded are those that are either infusible or produce a colouring effect upon the glass, for optical purposes the admixture of other glass-forming bodies than those which are intended to be present must be avoided on account of their effect in modifying the optical constants of the glass.
The metal is chiefly used, as the oxide, for colouring glass and porcelain.
Rhodes was again famous for its pottery in medieval times; this was a lustre ware at first imitated from Persian, though it afterwards developed into an independent style of fine colouring and rich variety of design.
The colouring agents are generally metallic oxides.
Since in all domesticated cats retaining the colouring of the wild species the soles of the hind-feet correspond in this particular with the Egyptian rather than with the European wild cat, the presumption is in favour of their descent from the former rather than from the latter.