THE RECOGNITION OF CHEMICAL CHANGES AS THE INTERACTIONS OF DEFINITE SUBSTANCES. The experimental study of combustion made by Lavoisier proved the correctness of that part of Stahl's phlogistic theory which asserted that all processes of combustion are very similar, but also proved that this likeness consists in the combination of a distinct gaseous substance with the material undergoing combustion, and not in the escape therefrom of the _Principle of fire_, as asserted by the theory of Stahl. After about the year 1790, it was necessary to think of combustions in the air as combinations of a particular gas, or _air_, with the burning substances, or some portions of them. This description of processes of burning necessarily led to a comparison of the gaseous constituent of the atmosphere which played so important a part in these processes, with the substances which were burned; it led to the examination of the compositions of many substances, and made it necessary to devise a language whereby these compositions could be stated clearly and consistently. We have seen, in former chapters, the extreme haziness of the alchemical views of composition, and the connexions between composition and properties. Although Boyle[7] had stated very lucidly what he meant by the composition of a definite substance, about a century before Lavoisier's work on combustion, nevertheless the views of chemists concerning composition remained very vague and incapable of definite expression, until the experimental investigations of Lavoisier enabled him to form a clear mental picture of chemical changes as interactions between definite quantities of distinct substances. [7] Boyle said, in 1689, "I mean by elements ... certain primitive and simple, or perfectly unmixed bodies; which not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixt bodies are immediately compounded, and into which they are ultimately resolved." Let us consider some of the work of Lavoisier in this direction. I select his experimental examination of the interactions of metals and acids.