After having been forgotten for more than half a century, the question of the nature of diamond was again raised by Jean d’ Arcet before the Académie des sciences in the 1770s. Quite surprisingly, diamond totally disappeared when heated to very high temperatures and, in order to understand this fascinating behaviour, Parisian chemists enthusiastically performed experiments from 1771 to 1773. They first used porcelain furnaces and then Tschirnhaus’ lens, an unsurpassed masterpiece constructed in Germany in approximately 1700. The analysis of handwritten laboratory notebooks, describing in detail the experiments conducted with this burning lens, reveals a little known close collaboration between Pierre-Joseph Macquer and Antoine-Laurent de Lavoisier. The successive steps of their research led to the revelation of the “true” nature of diamond, since it burned in air like common charcoal, releasing a gas that precipitated lime water. Contrary to the legend, Trudaine’s lens shed no additional light on the experimental evidence produced by the preceding collaborative academic research.
As early as the middle of the seventeenth-century burning mirrors and glasses were used for chemical operations such as calcination of antimony. Later, concave mirrors and convex lenses of about one metre in diameter, able to reach temperatures from 1400 to 2000°C at their focus, allowed a decisive progress in the knowledge of the nature of metals. This breakthrough led the mid-eighteenth century chemists, especially Rouelle and Macquer, to reconsider the Philosophers’ Stone and the alchemical dream of recomposing gold and silver. Macquer’s personal thoughts on the Philosophers’ Stone are reproduced here in full. Moreover, the behaviour of gold and ferruginous earths exposed at the focus of Tschirnhaus’ lens led Macquer to a hypothesis about the nature of solar rays that came surprisingly close to Homberg’s Principle Sulphur at the beginning of the century.