Upon the back of the furnace is a horizontal slit p, [fig. 71], which supports the fire-brick, s, and upon which the end of the muffle, if necessary, may rest; u, [fig. 71.], is the opening in the furnace where the muffle is placed.
The plan of the grate of the furnace is an ellipse: [fig. 73.] is a horizontal view of it. The dimensions of that ellipsis determine the general form of the furnace, and thickness of the grate. To give strength and solidity to the grate, it is encircled by a bar or hoop of iron. There is a groove in which the hoop of iron is fixed. The holes of the grate are truncated cones, having the greater base below, that the ashes may more easily fall into the ash-pit. The letter v, [fig. 71.], shows the form of these holes. The grate is supported by a small bank or shelf, making part of the furnace, as seen at a, [fig. 71.]
The ash-pit, C, has an opening y in front, [fig. 71.]; and is shut when necessary by the mouth-piece r, [fig. 70.] and [71.]
To give strength and solidity to the furnace, it is bound with hoops of iron, at b, b, b, b, [fig. 70.]
[Figs. 74.] [75.] [76.] are views of the muffle.
[Fig. 77.] is a view of a crucible for annealing gold.