The parallel usage of the two terms "alchemy" and "chemistry" by seventeenth-century writers has engendered considerable confusion among historians of science. Many historians have succumbed to the temptation of assuming that the early modern term "chemistry" referred to something like the modern discipline, while supposing that "alchemy" pertained to a different set of practices and beliefs, predominantly the art of transmuting base metals into gold. This paper provides the first exhaustive analysis of the two terms and their interlinguistic cognates in the seventeenth century. It demonstrates that the intentional partition of the two terms with the restriction of alchemy to the the sense of metallic transmutation was not widely accepted until the end of the seventeenth century, if even then. The major figure in the restriction of meaning, Nicolas Lemery, built on a spurious interpretation of the Arabic definite article al, which he inherited from earlier sources in the chemical textbook tradition. In order to curtail the tradition of anachronism and distortion engendered by the selective use of the terms "alchemy" and "chemistry" by historians, the authors conclude by suggesting a return to seventeenth-century terminology for discussing the different aspects of the early modern discipline "chymistry."