16 
MR. FARADAY ON THE 
the furnace, and carried away up the flue, so as to occasion no annoyance to the 
operator. The covers are slung- by a piece of platina wire, which being passed 
across the middle on the outside, is bent at each end round the edges, so that 
a rod of iron slightly curved at the extremity, easily suffices to remove them 
when the crucible is to be opened. Great care is always taken to put them in 
clean situations, and that in their removal nothing shall fall from them into 
the glass. 
27 . This furnace is found to be very effectual in its action; being connected 
with a high flue governed by a damper, great command of the temperature 
is obtained. The crucibles before being used are examined as to soundness, 
and then their temperature is raised gradually, and should not be above a dull 
red heat when the operation commences. The mixture already described 
(25) is then introduced, and the crucible covered ; decomposition of the 
nitrate of lead instantly commences ; the boracic acid loses its water, all the 
fixed elements unite ; and it is remarkable that though a considerable quantity 
of boracic acid usually sublimes with the water when the latter is driven off 
from its crystals unmixed with other substances, yet scarcely a trace seems to 
evaporate in the present instance, in consequence of the presence of the oxide 
of lead. 
28. The heat should not be raised too high or the operation hastened, and 
then the ebullition will proceed very gradually and favourably, the rough ma- 
terials being by degrees converted into glass. Before the first charge is entirely 
melted a second is put in, and when that is fused down, sometimes a third, 
according to the quantity of glass present and the soundness of the crucible. 
When all is fused, the temperature is allowed to rise, but not too much, 
lest action upon the crucible to a serious extent should occur ; the glass is 
then well agitated and mixed by a platina rake or stirrer, to be described 
hereafter. Finally the glass is either transferred by a platina ladle into trays 
roughly turned up out of old platina foil, or into a clean deep white earthen- 
ware vessel containing much distilled water. In the lattter case it is obtained 
in a divided state, and when drained, is dried on the sand-bath, and put up in 
clean bottles. 
20. When a crucible has been emptied of its first portion of glass, it will 
M i ve, if carefully used, for a second, third, fourth, or for many operations ; but 
