410 JOSEPH FOURIER. 
which is eminently worthy of being studied between the 
ball of iron at the ordinary temperature which may be 
handled at pleasure, and the ball of iron of the same 
dimensions which the flame of a furnace has very much 
heated, and which we cannot touch without burning our- 
selves. This distinction, according to the majority of 
physical inquirers, arises from a certain quantity of an 
elastic imponderable fluid, or at least a fluid which has 
not been weighed, with which the second ball has com- 
bined during the process of heating. The fluid which, 
upon combining with cold bodies renders them hot, has 
been designated by the name of heat or calorie. 
Bodies unequally heated act upon each other even at 
great distances, even through empty space, for the colder 
becomes more hot, and the hotter becomes more cold; 
for after a certain time they indicate the same degree of 
the thermometer, whatever may have been the difference 
of their original temperatures. According to the hypoth- 
eses above explained, there is but one way of conceiving 
this action at a distance ; this is to suppose that it oper- 
ates by the aid of certain effluvia which traverse space 
by passing from the hot body to the cold body ; that is, 
to admit that a hot body emits in every direction rays of 
heat, as luminous bodies emit rays of light. 
The effluvia, the radiating emanations by the aid of 
which two distant bodies form a calorific communication 
with each other, have been very appropriately designated 
by the name of radiating caloric. 
Whatever may be said to the contrary, radiating heat 
had already been the object of important experiments 
before Fourier undertook his labours. The celebrated 
academicians of the O’mento found, nearly two centuries 
ago, that this heat is reflected like light; that, as in the 
