_Carbon._--As has already been indicated, an admixture of carbon in some suitable form is essential in the case of certain glass mixtures. The carbon for this purpose may be used in the form of either charcoal, coke, or anthracite coal. Of these, charcoal is undoubtedly the purest form of carbon, but it is excessively expensive in this country. Coke varies very much in quality according to the coal from which it has been produced, but it always contains notable proportions of ash rich in iron, and also some sulphur. Anthracite coal can be obtained in a very pure form, containing considerably less ash than that found in most kinds of coke, and this is therefore probably the most convenient form of carbon for this purpose. CHAPTER IV. CRUCIBLES AND FURNACES FOR THE FUSION OF GLASS. For the successful production of substances which are formed by a process of fusion, the use of refractory materials of a proper kind is of great importance. In the production of glass the double difficulty has to be overcome of finding substances capable of being formed into furnaces and crucibles which shall not only resist the softening and melting action of the furnace heat for long periods of time, but shall also resist the dissolving action of the molten glass itself. The refractory materials employed in connection with glass-making thus fall into two distinct groups, members of one group being those which meet both of the above requirements and can therefore be used in positions exposed to direct contact with molten glass, while members of the second group are materials which resist the action of the heat and flame gases but cannot resist the dissolving effect of the glass itself; these, of course, can only be placed where molten glass is not liable to touch them. We shall deal with the former group first.