The elements of the fourth group are of greater importance in connection with glass. Carbon is capable of exerting powerful colouring effects when introduced into glass. These effects are of two kinds, viz., indirect in consequence of the reducing action of carbon on other substances present, and direct from the presence of finely-divided carbon or carbides in the glass. The latter are similar in kind to those produced by the presence of other finely-divided elementary bodies (copper, gold, lead, etc.) except that the lightness of the carbon particles tends to the production of yellow and brown colours rather than of red and black, while the chemical nature of carbon renders the glass in which it is suspended indifferent to rapid cooling, so far as the carbon tint is concerned. The indirect effects of carbon, in reducing other substances that may be present in the glass, become evident with much smaller proportions of carbon than are required to produce visible direct effects. As we have seen above, carbon, in the form of coke, charcoal or anthracite coal, is regularly introduced, as a reducing medium, into glass mixtures containing sulphate of soda. If even a slight excess of carbon be used for this purpose, the formation of sulphides and poly-sulphides of sodium and of calcium results, and these bodies, like all sulphides, impart a greenish-yellow tint to the glass, at the same time bringing other undesirable results in their train. _Silicon_, in the form of silicic acid and its compounds, is a fundamental constituent of all varieties of glass, and in this form is in no sense a colouring substance; on the other hand, there is no doubt that under some conditions silicon may be reduced to the metallic state at temperatures which normally occur in glass-furnaces, and it is practically certain, that if present in glass in this condition, silicon would colour the glass. It is just possible that some of the colouring effects produced in ordinary glass by powerful reducing agents, such as carbon, either in the solid form or as a constituent of furnace gases, may be due to the reduction of silicon in the glass. _Tin_ by itself does not appear to have any colouring effect upon glass, except that its oxide, in a finely suspended state, produces opalescence and, in large quantities, white opacity. Tin, however, is used in conjunction with copper in the production of copper-ruby, to which reference has already been made. _Lead and Thallium_ have already been dealt with, and it only remains to add that their presence in the glass, although not in itself producing any intense colouring action, increases the colouring effects of other substances. This is probably merely a particular case of the fact that dense glasses, of high refractive index, are more sensitive to colouring agencies than the lighter glasses of low refractive index; this applies to barium as well as to lead and thallium glasses.