The alchemists considered it possible to arrange all substances in four general classes, the marks whereof were expressed by the terms hot, cold, moist, and dry; they thought of these properties as typified by what they called the four Elements--fire, air, water, and earth. Everything, they taught, was produced from the four Elements, not immediately, but through the mediation of the three Principles--mercury, sulphur, and salt. These Principles were regarded as the tools put into the hands of him who desired to effect the transmutation of one substance into another. 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of the element oxygen; and for many years after Lavoisier, the alchemical expression _the principle of acidity_ was superseded by the word _oxygen_.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_. 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.