A case study of the Clausius-Williamson hypothesis sheds light on the development of the physical sciences during the nineteenth century. In the 1850s, Rudolf Clausius and Alexander William Williamson independently developed similar hypotheses at a time when physics and chemistry were beginning to be considered independent endeavors. Some thirty years later, after specialization took root, their names were associated; the two hypotheses became the hypothesis of Clausius-Williamson. How and why were these distinct investigations conducted in the 1850s unified in the 1880s? The current historiography addresses the Clausius-Williamson hypothesis as it is featured in subsequent interpretations by Svante Arrhenius, but does not thoroughly analyze the published writings of Clausius and Williamson themselves. This paper reappraises Clausius’s and Williamson’s works in their original context and analyzes how their hypotheses came to be associated. This case study emphasizes how the relationship between physics and chemistry evolved in the nineteenth century. More specifically, it underscores the limited communication between these disciplines in the 1850s and the rise of interdisciplinarity in the 1880s, which led to the creation of a new field: physical chemistry. From the study of the emergence and success of the theory of ionic dissociation and physical chemistry, I show that referring to the authoritative figures of Clausius and Williamson legitimized and valorized investigations at the borderlands of physics and chemistry in a context of increased specialization.