In this treatise Boyle sets out to prove that the number of the peripatetic elements or principles hitherto assumed by chemists is, to say the least, doubtful. The words “element” and “principle” are used by him as equivalent terms, and signify those primitive and simple bodies of which compounds may be said to be composed, and into which these compounds are ultimately resolvable. He considered that the matter of all bodies was originally divided into small particles of different shapes and sizes, and that these particles might unite into small “parcels,” not easily separable again; that a great variety of compounds may arise from a few ingredients; that various substances are obtainable from bodies by fire; that fire is not the true and genuine analyser of bodies, since it does not separate the principles of a body, but variously alters its nature; and that some things obtained from a body by fire were not its proper or essential ingredients. Three is not precisely and universally the number of the distinct substances or elements into which all compound bodies are resolvable by fire, inasmuch as some bodies afford more than three principles. Earth and water are as much chemical principles as salt, sulphur, and mercury. Even the limitation to five chemical principles is too narrow. Such is proved to be the case by the mode in which bodies, animals and vegetable, grow, and by the analysis of minerals and metals. The chemical theory of “qualities” of the Spagyrists is narrow, defective, and uncertain; supposes things not proved; is often superfluous, and frequently contradicts the phenomena of nature. The “principles” found in bodies cannot be the cause of their qualities, since contrary qualities are ascribed to the same body. He concludes, therefore, that the Paracelsian elements—their “salt,” “sulphur,” and “mercury”—are not the first and most simple principles of bodies; but that these consist, at most, of concretions of corpuscles or particles more simple than they, and possessing the radical and universal properties of volume, shape, and motion. [Illustration: ROBERT BOYLE. From a painting by F. Kerseboom in the possession of the Royal Society.]