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of the element oxygen; and for many years after Lavoisier, the alchemical expression the principle of acidity was superseded by the word oxygen.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.  Nevertheless it is convenient, indeed it is necessary, to place together many compounds which react similarly under certain defined conditions, and to give a common name to them all.  The alchemists also classified substances, but their classification was necessarily more vague than ours; and they necessarily expressed their reasons for putting different substances in the same class in a language which arose out of the general conceptions of natural phenomena which prevailed in their time.

The primary classification of substances made by the alchemists was expressed by saying; these substances are rich in the principle sulphur, those contain much of the principle mercury, and this class is marked by the preponderance of the principle salt.  The secondary classification of the alchemists was expressed by saying; this class is characterised by dryness, that by moisture, another by coldness, and a fourth by hotness; the dry substances contain much of the element Earth, the moist substances are rich in the element Water, in the cold substances the element Air preponderates, and the hot substances contain more of the element Fire than of the other elements.

The alchemists went a step further in their classification of things.  They asserted that there is One Thing present in all things; that everything is a vehicle for the more or less perfect exhibition of the properties of the One Thing; that there is a Primal Element common to all substances.  The final aim of alchemy was to obtain the One Thing, the Primal Element, the Soul of all Things, so purified, not only from all specific substances, but also from all admixture of the four Elements and the three Principles, as to make possible the accomplishment of any transmutation by the use of it.

If a person ignorant of its powers were to obtain the Essence, he might work vast havoc and cause enormous confusion; it was necessary, therefore, to know the conditions under which the potencies of the Essence became active.  Hence there was need of prolonged study of the mutual actions of the most seemingly diverse substances, and of minute and patient examination of the conditions under which nature performs her marvellous transmutations.  The quest of the One Thing was fraught with peril, and was to be attempted only by those who had served a long and laborious apprenticeship.