"Discorso sopra la Chimica is an early seventeenth-century manuscript on alchemy written by the Florentine priest Antonio Neri, best known as the author of the first published treatise on glassmaking - L'Arte Vetraria (1612) - which was widely read for centuries. The Discorso shows a different face of Neri, that of the alchemist with a profound knowledge of Paraselsian doctrine, dedicated to the transmutation of metals, and an advocate of iatrochemistry. This picture is apparently incompatible with that of the technical glassmaker and the champion of knowledge based on experience. However, even in his Discorso the author affirms the value of experimental practice, and the experimentum has the all-important benefit of legitimizing the validity of alchemical doctrines. Knowledge does not come from reading the books of the sages of antiquity, but from the “practice of many experiences.“ This emphasis on experimentation constitutes a unique feature of the Discorso. It was perhaps the `modernity' of Neri's discussion of alchemy that made the manuscript the object of plagiarism. Careful comparison reveals that an entire chapter of Prodromo, written by the Jesuit Francesco Lana Terzi in 1670, was copied from Neri. In the Discorso old and new are intertwined and validate one other, showing how Neri was a quintessential representative of his time, when scientific models that now appear irreconcilable coexisted, often forming a complex web. In this paper I discuss Antonio Neri and the background to his important work, reflecting on its impact and what it tells us about a fascinating and complex period in the birth of modern science. This is followed by a translation of the complete text of Discorso sopra la Chimica."