You may move the glass about; you may rotate these viscid fluids in a closed vessel; and you may even pour them, provided the last part of your stream does not fall on the poured-out mass.

To return to actual glass, the subject divides itself into two main systems of procedure, viz., you may make glass by the fusion of pure silica, lime, and potash, or other alkali, with or without the addition of a considerable quantity of oxide of lead, which is used where great density and refractive power are required. Then it becomes more desirable that an intimate mixture of the materials should take place, and throughout the twelve to twenty hours required for fusion, no subsidence of the heavy portions of the mixture, or flotation of the lighter ones, should be suffered to take place, or a homogeneous mass will not be obtained. It is manifestly easy to remove the heavy, sweet particles from the lower part of a cup of tea by one or two gentle movements of a spoon, and so get the whole cup of fluid equally sweet, but we have been warned by our oil experiment that the fluid glass must not be stirred by a rod or we introduce air; and if we wait long enough for the air slowly to find its way again to the surface, we inevitably have an interval in which the difference in specific gravity of the several materials will assert itself, and we get subsidence, lose the homogeneity of the mass, and all our stirring will have been in vain. The outcome of these and other observed conditions was the proposal to employ oscillating, semi-rotating, or continuous slow-rotating melting crucibles, the latter being preferred. The crucibles and the proper kind of furnace for this purpose may be largely varied; one of the simple forms is given in Fig. 24, Plate IX.; its leading features are representative of all the others.

The furnace there shown consists of a cylindrical casing of iron, A, lined with firebrick, B; it is divided into two chambers, the lower one, C, being provided with firebars on which the fuel rests; while the upper chamber, D, is cylindrical in form with a curved roof, having an opening at J formed in a circular piece of moulded firebrick J*, which is removed when putting in the rotating crucible H. Above the opening, J, is a suspended hood, E, which communicates with a tall chimney; the lower part of the chamber, D, is conical in form, having a small central opening, F, and four larger cylindrical openings, G, surrounding it, each of which communicates with the fire chamber, C, and allows the flame to ascend and play up and around the crucible, H. This crucible is conical in form both above and below its largest diameter, and terminates in a raised neck or mouth at H*; the furnace, A, is suspended on axes, occupying the position indicated by the letter M; these axes are fitted to a strong iron ring or hoop, N, which surrounds the furnace, and is itself supported on the axes, P, which rest on iron frames, O.

The axes, P, are placed at right angles to the axes, M, so as to allow the crucible to roll or gyrate on its axis, its upper and lower ends moving in a circular path. It will be observed that the crucible, H, rests on one of its sides on the conical floor of the chamber, D, and is kept in position by its lower spherical end, M, moving in the cylindrical opening, E. Now if the furnace be moved quietly on its axis, the crucible gravitating to the lowest inclined side of the conical surface on which it rests, will roll round on its own axis so long as the furnace is kept in motion. This motion of the furnace may be easily effected by means of a short-throw half-crank on a vertical axis passing upward through the floor in a line through the centre of the furnace, the crank-pin having a spherical end fitting into the cylindrical socket projecting downwards from the underside of the ashpit. The motion of the furnace should be very slow, so as to give about one revolution of the crucible in five or ten minutes, and thus allow a constant movement of the whole of the material to take place without dividing or breaking the continuity of the mass, preventing any subsidence of the heavier particles, and securing the perfect homogeneity of the whole. When the fusion and mixing is judged to be complete, the crucible can be pushed with a rod into an upright position, and, by drawing the fire, cooled as rapidly as possible by the current of air flowing through the furnace.

Homogenous optical glass, free from those long "wreaths" or lines of varying density, so common in ordinary glass, was also proposed to be made in the following manner. A large potful of glass of the required composition must be allowed to get cold, and then broken up, the central portions only being selected for use. These pieces are to be crushed, all the glass being reduced to absolute powder, and separated by sifting; all pieces exceeding the size of a grain of rice should be rejected. The very small and nearly equally-sized fragments that remain are then to be carefully washed in distilled water and put into a lenticular-shaped crucible, the exterior surface of which should be glazed, so as to render it impervious