A further consideration arises from the fact that chromic oxide is itself an extremely refractory body, and is therefore comparatively difficult to incorporate with glass, while its presence tends to make the glass itself more viscid and refractory; the simultaneous introduction of the alkali, as provided by the use of the bichromate, is thus an advantage in restoring the fluidity and softness of the glass when finished, while also facilitating the solution of the chromium in the glass during the fusion process; this process of solution, however, takes some time, chromium glasses being liable to appear patchy if insufficient time is given to the “founding.” _Uranium_ is one of the rarer and more costly elements, but is nevertheless used in glass-making for special purposes on account of the very beautiful fluorescent yellow colour which it imparts when added in small proportions. This yellow is quite characteristic and unmistakable, so that none of the other varieties of yellow glass can ever be used as a substitute for uranium glass, but the great cost of the latter prevents its extended use. Uranium is usually introduced into glass mixtures in the form of a chemical compound, such as uranyl-acetate or uranyl-nitrate, both these substances being obtainable in the form of small, intensely bright yellow crystals. _Fluorine_ occurs in a number of glasses in the form of dissolved or suspended fluorides, principally fluoride of aluminium. The element is not essentially a colouring substance, and is only mentioned here because the fluoride named is the most frequently used means of producing “opal” glass. The fluoride is most frequently introduced into the glass mixtures as calcium fluoride, used in conjunction with felspar, or as cryolite, a natural mineral which consists of a double fluoride of sodium and aluminium.