TRANSACTIONS. 
I—An Account of some Experiments on Radiant Heat, involving an extension 
of Prevost’s Theory of Exchanges. By BALFour STEWART, Esq. Communi- 
cated by Professor Forbes. 
(Read 15th March 1858.) 
Dwwision of Subject. 
1. This paper consists of two parts, the first of which is confined to describing 
the experiments performed ; while in the second it is attempted to connect these 
with certain theoretical views regarding Radiant Heat. 
2. The experiments were made with a fourfold object; at least, for the sake 
of clearness, it is well to class them into four distinct groups :— 
Group I. Contains those experiments in which the quantities of heat radiated 
from polished plates of different substances, at a given tempera- 
ture, are compared with the quantity radiated from a similar sur- 
face of lamp-black, at the same temperature. 
II. Those in which the quantities of heat radiated at the same tempe- 
rature, from polished plates of the same substance, but of differ- 
ent thicknesses, are compared with one another. 
III. Those in which the radiations, from polished plates of different sub- 
stances at any temperature, are compared with that from lamp- 
black at the same temperature, with regard to the quality or na- 
ture of the heat radiated. 
IV. Those in which the same comparison is made between the radiations 
from polished plates of the same substance, but of different thick- 
nesses. 
Instruments used, and Method of using them. 
3. 1 am indebted to the kindness of Professor Forses for the use of a delicate 
thermo-multiplier, consisting of the sentient pile, and its attached galvanometer 
and telescope ; as well as for much valuable information with regard to the proper 
method of using the apparatus. 
VOL. XXII. PART I. A 
