56 
MR. FARADAY ON THE 
Besides the iron covers, there are a set of earthenware covers, consisting of 
6 square tiles each If inch thick(62). These are notched to receive the handles 
of the iron covers, and being put together over them, constitute a covering of 
earthenware, which very importantly assists in retaining the heat. 
The tiles and brick used in the annealing process (89) are the ordinary dry 
varieties, with some pieces of various sizes, to allow of the close adjustment of 
the whole. 
The earthenware supporting blocks (5/) required for the arrangement and 
support of the platina tray should be formed out of some kind of flat unglazed 
ware containing as little iron as possible, and should be of various thicknesses, 
sizes, and forms, although parallelopipeds are the most usual. They should 
not be of such substance as is liable to fly or send oft' anything when heated ; 
and when any portion of glass adheres to them, it should either be cleared off or 
the piece thrown away. The Cornish tile before described (52. 53) is excellent 
for this use, and may be sawn, rasped or ground into any shape required. 
The glass covers (60, &c.) that have yet been used were merely inverted eva- 
porating basins. They answer the required purpose exceedingly well, except 
that, when large, they are too strong, too heavy, and too deep. Some covers 
for the purpose are therefore in progress, and as they only have to support their 
own weight and hold together, they are to be thin. The covers should be of 
very refractory and highly baked ware ; it may be desirable to have them very 
slightly glazed, to keep them clean, and prevent the absorption of any sub- 
stance which might send off vapours injurious to the glass. 
The lire tools required for this furnace will suggest themselves. Amongst 
the rest should be a pair of tongs which will readily lay hold either of the 
earthenware tile or the iron covers ; a slag and coke rake (89) ; and a stoking 
iron, with its extremity bent, for the purpose of breaking the clinkers off the 
bars from beneath upwards. 
Preparation of spongy platina. 
The platina used for this preparation should be pure, and may be the refuse 
pieces resulting from such plate and foil as has been in use for trays in former 
experiments. This, after being taken out of the pickle (93), and condemned 
as useless for other purposes in the glass-house, should be trimmed from all 
