The historical treatment of atomism and the mechanical philosophy largely neglects what I call "chymical atomism," namely a type of pre-Daltonian corpuscular matter theory that postulated particles of matter which were operationally indivisible. From the Middle Ages onwards, alchemists influenced by Aristotle's Meteorology, De caelo, and De generatione et corruptione argued for the existence of robust corpuscles of matter that resisted analysis by laboratory means. As I argue in the present paper, this alchemical tradition entered the works of Daniel Sennert and Robert Boyle, and became the common property of seventeenth-century chymists. Through Boyle, G.E. Stahl, and other chymists, the operational atomism of the alchemists was even transmitted to Antoine Laurent Lavoisier, where it became the basis of his claim that elements are simply "the final limit that analysis reaches."