MR. FARADAY ON THE 
5‘2 
the crucibles inserted through them may leave plenty of room for the inter- 
vention 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 to- 
bacco 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 sepa- 
rated by a short interval. 
The crucibles used are 5 inches high outside, 3| inches diameter at the top, 
and 2 inches diameter at the bottom. They are of pure porcelain biscuit, per- 
fectly 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 the % or ^rd 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 va- 
pours that are evolved from the mixture to pass away. 
The covers to the crucibles are evaporating basins about 4 \ inches in dia- 
meter. They arc slung with their edges downwards by a piece of platina wire 
sufficiently strong for the purpose, which being first bent at the middle into an 
angle, is then stretched across the outside of the basin, and has its ends bent 
round the opposite edges of the latter. The bent extremity of an iron rod 
passed under the loop thus formed over the middle of the bottom, serves to 
