Jouyv—lncandescent Electric Furnaces. 483 
In these furnaces the fireclay plays the part of a conductor of 
heat, but not sensibly, of electricity—distributing the heat 
produced on the wire evenly over the area of the vessel. It will 
probably be found that under prolonged usage—unless the clay is 
a nearly pure silica—the electrical resistance of the clay will 
diminish. But the change is a slow one, even in impure clays. 
A refractory clay, rendered coherent by silicate of soda (as supplied 
by Messrs. Fletcher, of Warrington), has successfully served many 
purposes to which I have applied these furnaces. A certain amount 
of cracking ensues with this clay after repeated heating; but the 
wire binding the parts together maintains the integrity of the 
crucible with but little loss of usefulness. 
The wire is best of platinum, and as it may be used over again 
on the wearing of the crucible, it is not a source of running 
expense. A diameter of about two-thirds of a millimetre is not 
too heavy. The temperature obtained must be found by melting 
substances of known fusibilities in the inner vessel. A chart of 
current and temperature may then be plotted, and will hold good 
for a considerable amount of use, but should be checked by 
occasional observations. ‘The regulation of the temperature is 
an easy matter, as any current regulator will accomplish it: 
