we have seen, particularly on the basis of the fact that light and heat propagate in the vacuum created in a pneumatic machine. This ethereal matter splits into a large number of subtle matters, which are invoked to explain various phenomena. For example, there is magnetic matter, responsible for the orientation of the magnetic needle and for the attraction between the magnet and iron, which operates in a vacuum, as Boyle’s experiments have shown; or electrical matter, responsible for the glow and the effects of attraction and repulsion between electrified bodies, which is transmitted through the vacuum, as attested by the sparks obtained by Francis Hauksbee in his static electricity generator. Some of these matters are supposed to be identical, for example, those that transmit light and heat, according to Newton. Nollet equated electrical matter with fire matter. Laurent Béraud tried to unify magnetic matter and electrical matter. Many theories were developed to achieve a more integrated vision of ether and subtle matter. Most of the above-mentioned matter, as well as some others, such as Cassini’s refractive matter, Descartes’ solar matter or the subtle air invoked by Jean Bernoulli to explain the mercurial phosphorus (luminous barometer), take part in the representations of the atmosphere which were developed at the end of the 17th century and in the 18th century to explain the natural phenomena being used to estimate its height: the refraction of starlight (refractive matter), the reflection of sunlight (the effect of the solar atmosphere, and therefore of solar matter, on the height of the atmosphere estimated by the twilight method), fiery meteors passing through the atmosphere and falling stars (electrical matter), the aurora borealis (solar matter, subtle air, magnetic matter), etc. The purpose of the following chapters is specifically to examine these subtle matters, and the way they are supposed to interact with the atmosphere to explain the different phenomena that define the quest to determine its height.
