_Gold_ is introduced into glass for the production of brilliant ruby tints; its behaviour is very similar to that of copper, except that the noble metal has a great tendency to return to the metallic state without the aid of reducing agents. No addition of tin is therefore required, but the rate of cooling, etc., must be properly regulated, since rapidly cooled glass containing gold shows no special colour, the rich ruby tint being only developed when the glass is re-heated and cooled slowly. The colouring effect of gold is undoubtedly more regular and uniform than that of copper, and it is accordingly possible to obtain much lighter shades of red with the aid of the noble metal. “Gold ruby” can therefore be obtained of a tint light enough to be used in sheets of ordinary thickness, and the process of “flashing” is not essential. The elements of the second group, such as magnesium, calcium, strontium, barium, zinc and cadmium, exert no strong specific colouring action on glass, with perhaps the exception of cadmium, and that element only does so to any considerable extent in combination with sulphur, sulphide of cadmium having the power of producing rich yellow colours in glass. The sulphur compounds of barium also readily produce deep green and yellow colours, and the formation of these tints is, indeed, very difficult to avoid in the case of glasses containing much barium. A colouring effect has sometimes been ascribed to zinc, but this is not in accordance with facts. Of the elements of the third group, only boron and aluminium are ever found in glass in any notable quantity. Boron is present in the form of boric acid or borates, and as such produces no colouring effect, nor does there seem to be any tendency for the separation of free boron. The compounds of aluminium also possess no colouring effect, although certain compounds of this element are utilised for imparting a white opacity to glass for certain purposes--such glass being known as “opal.”