2 75 
on the Production of Light and Heat. 
ware tube, along with a gold one, and plunged into melted 
glass, is much longer in heating than the gold one. This is 
easily explained, upon a well known principle, namely, that 
in two bodies of unequal temperature, the colder body con- 
ducts the heat from the hotter at a rate directly proportionate 
to their difference of temperature. Now the surface of the 
earthen ware cylinder, as the heat is conducted very slowly 
from it by the interior mass, soon becomes very nearly of the 
temperature of the hot glass contiguous to it. The surface of 
the gold cylinder, on the contrary, having its heat conducted 
from it much faster by the interior mass, is of consequence dis- 
posed to receive the heat with greater rapidity. 
EXPERIMENT V. 
Equal pieces of gold, silver, copper, and iron, blackened all 
over, and suspended by a wire in a red hot crucible, became 
red in the order in which they are here set down ; and when 
made equally red, and removed into the dark, they disap- 
peared in the same order. When just brought out of the fire, 
they all looked equally red ; but when they had cooled a little, 
the iron was much the brightest. 
An earthen ware cylinder, tried with the metals, disappeared 
much sooner than any of them, the interior part not com- 
municating its heat quick enough to keep the surface of the 
temperature of red heat : accordingly, when broken, though 
the surface gave no light, the mass was luminous internally. 
From a parity of reasoning, a gilt earthen ware cylinder, 
suspended in a red hot crucible along with a gold one, would 
probably become red on the surface before the gold one. 
