title SUMMARY /title The spiritus salis (hydrochloric acid) was known as a mineral acid from antiquity, but its exact chemical composition remained a mystery until the emergence of electrochemistry in the early nineteenth century. The case of this acid shows the complex status of the concept of chemical substance in the various phases of the historical evolution of modern chemical thought. The aim of this paper is to show such a complex status through an examination of the research in electrochemistry which took place in Tuscany at the beginning of the nineteenth century. The paper specifically examines the works of Francesco Pacchiani who, in 1805, thought to have proved that spirit of salt was composed of hydrogen and oxygen. Between 1805 and 1806 the international scientific community carefully discussed Pacchiani's research and discoveries . The Pacchiani case reveals that the composition of the spirit of salt was a true puzzle for chemical philosophy at that time. The study of the fate of Pacchiani's work also provides some information on the Italian chemical community at the time of Napoleon I and demonstrates that Giovanni Fabbroni, the vice-director of the Museum of Physics and Natural History in Florence, was the leading representative of Tuscan science and was much admired throughout Europe. Fabbroni took a prominent part in the affairs of the Pacchiani case. Therefore the paper offers a primary documentation of Fabbroni's ideas on electrochemistry and of his efforts to favour the spreading of Tuscan research in this field.