At the end of the 18th century, instrumental music, formerly subordinate to vocal music and shackled to the doctrine of imitation, dramatically emerged as a new and powerful form or art, capable of expression. Many scholars today turn to developments in aesthetic philosophy--the birth of German Idealism, "absolute music," or Kantian formalism-- to explain the changing perception of instrumental music. Such explanations, though they illuminate important aspects of contemporary philosophy, ultimately blind us to fascinating developments in musical practice. This dissertation locates the heart of this transformation not in philosophical aesthetics, but in the musical medium itself, specifically focusing on the birth of the concept of timbre and the ensuing transformations to musical discourse. Tracing the concept of timbre from its birth in the writings of Rousseau through its crystallization in the early 19th century with the emergence of "orchestra machines" and a widespread obsession with effect, the dissertation explores the impact of the new focus on the musical medium in different registers of musical culture. The project examines the use of the metaphor of color borrowed from painting and Newtonian science, the philosophical attitudes towards transience and sensation in the writings of Kant and Herder, ideas of composition and orchestration in music treatises, and composers' new uses for the orchestra through close analysis of Haydn's style of orchestration in the 1790s. In addition, the dissertation draws upon as resources many now-forgotten instruments that were invented in this period. Celebrated in their day, these instruments serve as invaluable repositories of the sonorities that captured the 18th- and early 19th-century ear. These changes in musical practice were fueled by the solidification of the orchestra as a concept, musical body, and institution. Whereas earlier critics likened instrumental sonorities to random paint splatters, later thinkers emphasized the individual character and inherent expressive capacity of each instrument. Only after this radical reevaluation of its foundations could music begin to be recognized as a means to connect with the human heart and mind.