Calx, definition

The nomenclature reform of the French chemists was of fundamental importance, replacing the old body-spirit terminology with new terms, based on oxygen. The calx was now termed the oxide, and the spirit of vitriol became sulfuric acid. The assumption that oxygen was the acid generator, as its name implied, was flawed, but the systematic nomenclature based on the increase of acidity with increase of oxygen content lived on, e.g. the acidic component of the sulfides, sulfites, and sulfates. The definition of a chemical element as the latest term whereat chemical analysis has arrived ... 

Boyle s experimental work in chemistry did not consist of a string of haphazard operations, like that of his predecessors. He worked to a plan, directed towards the solution of a definite problem. He did not, of course, always arrive at the correct interpretation of his observations. Notably, he attributed the increase of weight in his experiments on the calcination (heating in air) of metals to their absorption of igneous particles, as mentioned above so that he viewed the resulting calx (oxide) as a compound of the metal with fire. Here, he had fastened his attention upon a problem of the very first importance in chemistry, namely, the chemical nature of combustion, or burning. 

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