crucibles. This plate is formed with circular holes, about 3 inches, or rather more, in diameter, arranged as in fig. 1, that the crucibles inserted through them may leave plenty of room for the intervention of coke and flame. As many round crucible covers belong to the plate as there are holes, serving to close such of them as are not occupied by crucibles.
As the plate becomes very hot when in use, it is necessary to have a second above it, which may be formed of sheet-iron with corresponding holes, and when put into its place, separated from the first, a little space, by pieces of tobacco pipe, or other convenient substance, to include a layer of air. But it is much better for the retention of heat, and also for its superior cleanliness, that this second plate should consist of pieces of earthenware fitted to each other, so as to cover the surface of the iron plate, from:which it should also be separated by a short interval.
The crucibles used are 5 inches high outside, 3¼ inches diameter at the top, and 2 inches diameter at the bottom. They are of pure porcelain biscuit, perfectly white and clean. They should be made as thin as possible, of the finest and most refractory kind of ware, and baked at a high temperature. We have some crucibles made about thirty years ago for Mr. Hatchett, which, though not of the size required, are precisely the right kind of ware. They have been used many times in succession without cracking or being importantly acted upon by the glass, and no sensible degree of impurity was given to it from them.
When these crucibles are arranged in the furnace, they should be supported by little stands of earthenware, formed out of brick or Cornish tile, so that their edges shall rise about ½ or ⅓rd of an inch above the surface of the upper covering plate, that no impurity may enter them. The holes in the plate should be of such dimensions that, when hot, the crucibles may lit loosely, that they may be uninjured, and also that there may be room between for the vapours that are evolved from the mixture to pass away.
The covers to the crucibles are evaporating basins about 4½ inches in diameter. They are slung with their edges downwards by pieces of platinum wire sufficiently strong for the purpose, which being first bent at the middle into an angle, are then stretched across the outside of the basins, and have their ends
