Antoine Lavoisier

1. Chemistry before Lavoisier

2. chemistry became a three-dimensional subject.

Unlike astronomy and mechanics, whose subject matter had been transformed in the 16th and 17th centuries by Copernicus, Galileo, Newton and others, chemistry retained older patterns of thought well into the 18th century. For example, many chemists still did not think of chemical changes as due to interactions of a limited number of elements, or as involving rearrangements of groups of atoms. Certain substances could burn, or were acidic in taste, was explained by supposing that they contained a “burning” or “acid” principle.

These ideas together with a good deal of observational and experimental evidence to support them, were codified at the beginning of the 18th century in an influential theory of matter published by the German chemist Georg Stahl (1660-1734).

It was not until 1727, when the English clergyman Stephan Hales (1677-1761) showed that large volumes of “air” could be released from the pores of solids and liquids, that

3. The Stimulus of Guyton

In the spring of 1772 Lavoisier read an essay on phlogiston by a Dijon lawyer and part time chemist, Louis-Bernard Guyton de Morveau (1737-1816), where Guyton showed in an experiment that all his tested metals increased in weight when they were roasted in air. Most members of French Academy, including Lavoisier, thought that Guyton’s experiment was absurd and Lavoisier immediately saw a more likely explanation that, somehow, air was being “fixed” during the process of combustion and that this air caused the increase in weight. It followed that the “fixed air” should be released when the “calces” of metals were decomposed.

Lavoisier was able to verify this in October 1772 by using a large burning lens belonging to the Academy. When litharge (an oxide of lead) was roasted with charcoal an enormous volume

4. Discovery of “Dephlogisticated air” or oxygen

Two things led Lavoisier to change his mind about phlogiston. Firstly by Pierre Bayen (1725-1798) , a French pharmacist, to the fact that, when heated, the calx (oxide) of mercury decomposes directly to the metal mercury without the addition of charcoal. This made it difficult to see phlogiston theory be right.

The mercury calx had also come to the attention of Joseph Priestley. In August 1774 Priestley heated the calx in an enclosed vessel and collected a new “Dephlogisticated air” which he eventually found supported combustion far better than ordinary air did.

5. The new chemistry

Lavoisier was now in a position to bring about a revolution in chemistry by ridding it of phlogiston and by introducing a new theory of composition. He concluded:

“All these reflections confirm what I have advanced, what I set out to prove [in 1773] and what I am going to repeat again. Chemists have made phlogiston a vague principle, which is not strictly defined and consequently fits all the explanations demanded of it. Sometimes it has weight and sometimes it has not; sometimes it passes through the pores of vessels, sometimes they are impenetrable to it. It explains at once causticity, transparency and opacity, color and the absence of colors. It is a veritable Proteus that changes its form every instant!”