perhaps the commonest of all chemical changes, the question became narrowed, and chemists eagerly sought for an answer to the query, "What happens when a chemical substance burns?"
In those days natural phenomena were referred to the presence of "principles" or "essences" in the matter exhibiting the phenomena. A new principle was added to the list; and the question was supposed to be solved by saying that combustible substances are characterized by richness in phlogiston (Gr. phlogizō burn, or inflame), and that when they burn they lose this principle, so that the burned substance, or calx, consists of the original substance minus phlogiston.
The phlogisteans seem to have regarded their hypothetical principle as a modified form of fire, as fire confined in a material substance; but, as they gave no definition of fire, beyond saying it was one of the four elements, it was scarcely to be expected that they should define phlogiston. By restoring phlogiston to the burned substance, said the theory, the original matter is regenerated. Some substances, e. g., charcoal, are especially rich in phlogiston, and metallic calces may be converted into metals, i. e., may be unburned, by heating them with charcoal. Thus the phlogisteans regarded the phenomena which they studied in a purely qualitative manner: they asked only, What does this or that substance do under given conditions? not being aware that a full answer to this question can only be given when the other question—How much of some given effect is produced by a given quantity of this body under stated conditions?—had been answered.
The introduction and use of the balance carried the day in favor of those who opposed phlogistic views. If a substance loses something when it burns, it must weigh less than before burning—as a fact it weighs more—therefore it has not lost but gained something.
"Nay," replied the phlogistean, "it has lost something, but the weight of this something can only be expressed by a negative quantity."
"But a something with such properties is an absurdity," replied the opponent; "therefore it has no existence, and therefore your theory is utterly false."
The anti-phlogistean triumphed, and the principle of levity was banished from chemical science. But the principle returned in a modified form. Lavoisier, who opposed the Beccherian theory of phlogiston with signal success, himself propounded a theory of the constitution of solids, liquids, and gases, in which the "subtle principle" "caloric" played an important part. Lavoisier regarded oxygen as what he termed "concrete oxygen" plus a something—caloric; indeed, he appears to have looked on all substances in the concrete state as solids, and to have supposed that the addition of a certain quantity of caloric to these caused them to become liquids, while the addition of a further quantity of caloric produced gases.
