The properties of opium have also been known from early times to the Persians, who flavoured the drug with aromatics, and held it in great esteem. By them it was commonly called Theriaka. It is supposed to have been first introduced to China by the Arabs, who traded with the Chinese as early as the ninth century. Towards the end of the eighteenth century a trade sprang up with India, which rapidly increased, till it led to political difficulties, culminating in the war of 1842, and the signing of the treaty of Nanking, after which five ports of China were opened to foreign trade, opium being admitted as a legalised import in 1858. Opium smoking in China was practised in the seventeenth century, and gradually extended over the entire empire, and at the present time is almost a recognised habit among the people. With regard to the introduction of opium into India, the Mahomedans once having established its use began to make it a source of income. The Great Mogul monopolized the opium production and trade, and derived an immense income from the sale of the monopoly. With respect to its use in India, it is not easy to state with certainty whether or not and in what periods, it has increased over the various parts of the country. From the most recent reports it appears that "the largest amount of opium is produced in the central tract of the Ganges, extending from Dinapore in the east, to Agra in the west, and from Gorakhpur in the north to Hazaribagh in the south, and comprising an area of about 600 miles long and 200 miles broad." In the district of Bengal, the Government has the monopoly of the opium industry, and the districts are divided into two agencies, Behar and Benares, which are under the control of officers residing in Patna and Ghazipur. In 1883 the amount of acres under poppy cultivation was in Behar 463,829, and in Benares agency 412,625; but the export of opium has somewhat diminished since then. Any one may undertake the industry, but cultivators are obliged to sell the opium exclusively to the Government agencies, at a price which is fixed beforehand by the officials. The Government sells the ready goods to merchants at a much higher price, which difference is paid by the country to which the opium is exported. In India itself, the sale of opium is restricted to licensed shopkeepers, a practice which has proved to be useful, because in some places, when the licensed shops have been closed, a greater number of unlicensed and secret shops have sprung up, and have made the contract insufficient.