l, the oxidation
has proceeded slowly, but steadily, for some time, until the heat
evolved has raised the mass to the temperature of ignition.

The explanation of the phenomena of combustion was attempted at very
early times, and the early theories were generally bound up in the
explanation of the nature of fire or flame. The idea that some
extraneous substance is essential to the process is of ancient date;
Clement of Alexandria (c. 3rd century A.D.) held that some "air" was
necessary, and the same view was accepted during the middle ages, when
it had been also found that the products of combustion weighed more than
the original combustible, a fact which pointed to the conclusion that
some substance had combined with the combustible during the process.
This theory was supported by the French physician Jean Ray, who showed
also that in the cases of tin and lead there was a limit to the increase
in weight. Robert Boyle, who made many researches on the origin and
nature of fire, regarded the increase as due to the fixation of the
particles of fire. Ideas identical with the modern ones were expressed
by John Mayow in his _Tractatus quinque medico-physici_ (1674), but his
death in 1679 undoubtedly accounts for the neglect of his suggestions by
his contemporaries. Mayow perceived the similarity of the processes of
respiration and combustion, and showed that one constituent of the
atmosphere, which he termed _spiritus nitro-aereus_, was essential to
combustion and life, and that the second constituent, which he termed
_spiritus nitri acidi_, inhibited combustion and life. At the beginning
of the 18th century a new theory of combustion was promulgated by Georg
Ernst Stahl. This theory regarded combustibility as due to a principle
named phlogiston (from the Gr. [Greek: phlogistos], burnt), which was
present in all combustible bodies in an amount proportional to their
degree of combustibility; for instance, coal was regarded as practically
pure phlogiston. On this theory, all substances which could be burnt
were composed of phlogiston and some other substance, and the operation
of burning was simply equivalent to the liberation of the phlogiston.
The Stahlian theory, originally a theory of combustion, came to be a
general theory of chemical reactions, since it provided simple
explanations of the ordinary chemical processes (when regarded
qualitatively) and permitted generalizations which largely stimulated
its acceptance. Its in