In setting chemistry on the proper track for fruitful development, one of Lavoisier's main contributions was to systematize the idea of elements at a time when some scientists were still struggling to understand substances in terms of their content of Air, Earth, Fire, and Water.
Admire the common sense of the empirical, that is experimentally-based, definition that he gives in the Preface to "Elementary Treatise of Chemistry" (1789) pp. xvii-xviii:
" Thus I will be content to say that if by the name of elements we mean to designate the simple, indivisible molecules that make up substances, it is probable that we do not know what they are : but if, on the contrary, we associate with the name of elements, or of the principles of substances, the idea of the furthest stage to which analysis can reach, all substances that we have so far found no means to decompose are elements for us; not that we can be certain that these substances that we regard as simple are not themselves composed of two or even of a greater number of principles, but because these principles never separate, or more precisely that we have no means to separate them, they behave with respect to us like simple substances, and we must not suppose them to be compound until the moment when experiment and observation furnishes us the proof."
(1) Do our chemical elements satisfy Lavoisier's definition?
(2) Consider Lavoisier's concern with the "radical of sea salt" from the point of view of his definition.
Below is Lavoisier's Table of Simple Substances (that is Elements). Note that while it begins with the "imponderable" (unweighable - no mass) elements "light" and "caloric", the other elements are ones we currently recognize as such. Furthermore notice how apt Lavoisier's new names are: "Oxygen" makes acid (his new theory) ; "Azote" without life, does not support respiration ; "Hydrogen" creates water (upon oxidation). His theory had trouble with the root (or radical, or base) of the acid of sea salt, muriatic acid.
The following table named the "compound radicals", the bases combining several elements which upon oxidation would yield complex acids. Many of these were organic acids that were discovered and purified by Scheele and would play key roles in the development of organic chemistry theory - particularly tartaric and benzoic acids.
(Note that Lavoisier created the -ous and
-ic suffixes and that
many of his "Unknown" theoretical predictions did not work out,
e.g. consider the 7th row on the "Muriatic radical".)