Albertus Magnus had advanced the opinion that the metals were earthy substances impregnated with a certain inflammable principle; but Beccher supported the idea of this principle not only as the cause of metallization, but likewise of combustibility. Stahl, however, one of the most extraordinary men that Germany ever produced, having adopted and amplified this theory, carried off the entire credit of being its founder, and it is universally spoken of as the _Stahlian Theory_. This theory forms so important a feature in the history of chemistry, and so long maintained its ascendency in the schools, that it will be necessary to give the reader a short summary of its principles. It assumed that all _combustible_ bodies are compounds: one of the constituents being volatile, and therefore easily dissipated during the act of combustion; while the other, being fixed, constantly remained as the residue of the process. This volatile principle, for which Stahl invented the term _Phlogiston_, was considered as being identical in every species of combustible matter; in short, it was supposed that there was but one principle of combustibility in nature, and that was the imaginary phantom Phlogiston, which for nearly a century possessed the schools of Europe, and, like an evil spirit, crossed the path of the philosopher at every step, and by its treacherous glare allured him from the steady pursuit of truth; for, whether a substance were combustible or not, its nature could never be investigated without a reference to its supposed relations with Phlogiston; its presence, or its absence, was supposed to stamp a character upon all bodies, and to occasion all the changes which they undergo. Hence chemistry and combustion came to be in some measure identified; and a theory of combustion was considered the same thing as a theory of chemistry.