1S29.] On the Manufacture of Optical Glass. 285 



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 cleanli- 

 ness, that this second plate should consist of pieces of earthen- 

 ware 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, 3^ inches dia- 

 meter 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 pre- 

 cisely 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 

 ,T or yi'd 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 fit 

 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 4^ 

 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 



