The Principles were not thought of as definite substances, nor as properties of this or that specified substance; they were considered to be the characteristic properties of large classes of substances.
The chemist of to-day places many compounds in the same class because all are acids, because all react similarly under similar conditions.
It used to be said that every acid possesses more or less of _the principle of acidity_.

Lavoisier changed the language whereby certain facts concerning acids were expressed.

He thought that experiments proved all acids to be compounds of the element oxygen; and for many years after Lavoisier, the alchemical expression _the principle of acidity_ was superseded by the word _oxygen_.

Although Lavoisier recognised that not every compound of oxygen is an acid, he taught that every acid is a compound of oxygen.

We know now that many acids are not compounds of oxygen, but we have not yet sufficient knowledge to frame a complete definition of the term _acid_.