96 
Proceedings of the Royal Society. 
taking place in about 12 seconds after the substance had been taken 
out of the boiling water apparatus, it was found that during this 
small portion of time the substance might be supposed to keep its 
original temperature of 212°, its cooling being so small as to be ne- 
glected. 
In order that different substances might be compared with one 
another, the same amount of heated surface was always presented 
to the pile. 
In the first group of experiments, the quantities of heat radiated 
from polished plates of different substances (heated to 212°) were 
compared with the quantity radiated from a similar surface of lamp- 
black at the same temperature. It was found that glass, alum, se- 
lenite, and thick mica, radiated very nearly as freely as lamp-black ; 
while the radiation from rock-salt was only 15 per cent, of that from 
lamp-black. 
In the second group of experiments, the quantities of heat ra- 
diated at 212° from polished plates of the same substance, but of 
different thicknesses, were compared with one another. 
It was found that thickness made a scarcely perceptible differ- 
ence on the quantity of heat radiated by glass, a somewhat greater 
difference on the quantity radiated by mica, and a very sensible 
difference on the quantity radiated by rock-salt — a thick plate of 
this substance giving more than a thin plate, in the proportion of 
nearly 5 to 3. 
The third group of experiments showed that heat from a polished 
plate of any substance is less transmissible through a screen of the 
same substance than heat from lamp-black ; this difference being 
exceedingly marked in the case of rock-salt, — the same rock-salt 
screen which transmits •Jths of the rays which fall upon it from 
heated lamp-black, transmitting only ^d of the rays that issue from 
heated rock-salt. 
The fourth group of experiments showed that heat from a thick 
plate of any substance is more transmissible through a screen of the 
same substance than heat from a thin plate. 
These four groups of experiments show that the radiation from 
diathermanous bodies, such as rock-salt, is much less copious than 
that from bodies of an opposite nature, such as glass ; and also that 
the radiation from diathermanous bodies increases with the thickness 
of the plate. 
