26 
MR. FARADAY ON THE 
Slate, however carefully heated, shivered and split, not only across, but parallel 
to its structure, and then, as soon as air intervened, it transmitted too little 
heat. It also softened, became curved, and let in air and smoke, and at last 
gradually fused, becoming unable to bear the weight of a large experiment. 
Yorkshire stone rubbed down into plates J-ths of an inch thick, answered mode- 
rately well, if the application of heat was carefully made and gradually raised. 
It cracked in a few places, but did not fall to pieces ; and it was more difficult 
of fusion than the former substances. Fire tiles of various kinds were tried ; 
those made of Stourbridge clay answered the best, and when about fths of an 
inch thick and carefully heated, might be successfully used; but that which we 
finally arrived at was the use of plates made of the materials from which 
Cornish crucibles are manufactured. These we obtained through the interven- 
tion of our President ; they were purposely manufactured for us by Mr. Michell 
of Caleneck in Cornwall, a gentleman who has been ever willing and anxious 
to assist us in our inquiries, by supplying us with vessels of any size or form, 
or any other article which it was in his power to produce. 
53. The Cornish plates have not much cohesion, and feel tender in the hand. 
They may be rubbed down to a flat surface, and resist any heat which can 
be applied to them in these or in much more powerful furnaces. They are 
therefore readily brought to any thickness, and when of about |ths of an inch, 
and supported in the furnace as before described (47), have strength to bear any 
weight required to be placed upon them. They do not crack, nor do they force 
themselves to pieces by expansion ; but they are porous, as indeed are in a 
greater or smaller degree all the materials of which the chamber and its sides 
are now composed. 
54. The porosity of these materials was of great importance; for it allowed 
of the passage of gaseous matter, and that even of a reducing nature, from the 
lire into the chamber. I have frequently had evidence that the sides and bot- 
tom might be considered as a very sieve-like partition between the fire, the 
flue, and the space called the chamber; for when the upper aperture has been 
closed, there has been a current through the chamber in the direction of the 
flame, the gaseous matter entering at the extremity nearest the fire, and pass- 
ing out at the end towards the flue. In one or two cases, oxide of lead was 
actually reduced, and the glass thus rendered cloudy. 
