In 1685 the Kangxi emperor legalized private maritime trade along the coast, establishing a series of customs stations in major port cities. The customs station at Canton became by far the most active in foreign trade and by the late Kangxi reign more than forty mercantile houses specializing in trade with the West had appeared. The Yongzheng emperor made a parent corporation comprising those forty individual houses in 1725 known as the Cohong system. Firmly established by 1757, the Canton Cohong was an association of thirteen business firms that had been awarded exclusive rights to conduct trade with Western merchants in Canton. Until its abolition after the Opium War in 1842, the Canton Cohong system was the only permitted avenue of Western trade into China, and thus became a booming hub of international trade by the early eighteenth century. By the eighteenth century the most significant export China had was tea. British demand for tea increased exponentially up until they figured out how to grow it for themselves in the hills of northern India in the 1880s. By the end of the eighteenth century tea exports going through the Canton Cohong system amounted to one-tenth of the revenue from taxes collected from the British and nearly the entire revenue of the British East India Company and until the early nineteenth century tea comprised ninety percent of exports leaving Canton.