Prevost's Theory of Heat Exchange
Once upon a time it was thought that only 'hot' bodies emitted radiation, but of course 'hot' is a relative term. Molten lava will feel burning hot to the skin, and ice will feel cold, but if you touch someone, they might feel your skin as either warm or cool.
Prevost suggested that all bodies radiate energy, but hotter bodies radiate more heat than colder bodies. Suppose for example that we have two bodies 1 and 2 at different temperatures
and
with 
Each body will then radiate heat to the other but since
the energy radiated from 1 to 2 is greater than the energy radiated from 2 to 1.
Heat will be transferred from body 1 to body 2 as long as
Eventually the bodies will be at the same temperature so
and heat will be transferred from body 1 to 2 at the same rate as from 2 to 1. The bodies will then be in thermal equilibrium.
