24 
MR. FARADAY ON THE 
40. The chamber was at first of cast iron, that material being selected as 
one which would bear a sufficient temperature without melting, would conduct 
and transmit the heat freely to the substances within, and could be easily 
obtained of the requisite form. The upper aperture was closed by plate iron 
covers, and in the first trials all appeared to answer well ; but when large 
experiments were made, and the heat was continued for a long time, the 
bottom gave way and became irregular; and upon endeavouring to rectify this, 
and place the tray of glass level by means of sand, the transmission of heat 
to the glass was prevented, the temperature of the iron rose, and the bottom 
melted. Besides these injurious liabilities, if the smallest portion of glass 
passed out of the tray, the moment it touched the iron it was reduced, the 
lead immediately caused fusion of the platina, and in an instant the tray was 
destroyed, the experiment stopped, the glass rendered black and useless, and 
the bottom of the chamber covered with lead and rendered unfit for another 
operation. 
50. Finally, one very curious action of the iron was discovered, which im- 
mediately caused its rejection. Plates of glass, which seemed very good in 
other respects, were frequently so discoloured by dark smoky clouds as to be 
useless. These could not be referred to any impurity which had been left in 
the materials or had entered accidentally, and, as the platina was in all such 
cases altered and injured, was at first supposed to be occasioned by some par- 
ticular action exerted between it and glass at high temperatures. But upon 
every fair trial to verify such chemical action, the proofs failed, however high 
the temperature used, or however minutely the metal was divided. At last the 
cause was discovered. To understand it, it must be known that the platina tray, 
with the glass in it, was either placed directly upon the bottom of the iron pan, 
or, for greater security, with only a plate of platina intervening; and that the 
whole was covered by an evaporating basin turned upside down, forming a sort 
of inner chamber within the large one. In this confined state the oxygen of 
t he portion of air present was soon abstracted by the heated metal, an oxide of 
iron being formed in consequence, and at the same time also, a portion of 
carbonic oxide from the carbon in the cast iron. At the high temperature to 
which the experiment was raised, this carbonic oxide was competent to reduce a 
portion of the oxide of lead in the glass to the metallic state, itself becoming 
