A handful of factors lead to a fast population growth during the mid of Qing period. The first source for the population growth was of course the economical prosperity under the century of the three Emperors Kangxi, Yongzheng and Qianlong (abbreviated to Kang Yong Qian Sanchao 康熙雍正乾三朝). During the whole course of Chinese history, the lack of arable land in the densely populated areas made it necessary to invent new techniques of agriculture to harvest as much as possible from a small amount of land. In the 18th century, Chinese agriculture was the most advanced of the world - but the cheapness of labour force in a densely populated land was an impediment for the widespread use of machinery on the countryside - until nowadays. New fruits from the Americas helped the Chinese population to obtain a better nourishment: potoatoes (tudou 土豆), peanuts (huasheng 花生), sorghum millet (gaoliang 高粱), corn (mais; yumi 玉米 or bangzi 棒子). Additionally, crops that can be used in industrial agriculture (plantations), like tea (cha 茶; Fujian dialects: dé), cotton (mian 棉), and sugar cane (jian 薦), stimulated private entrepreneurship and employment. Until 1770, the tax for the small peasants was the lowest of the whole history of China, and the whole countryside during the mid-Qing period seemed to be blessed with a relative high living standard and an education system that allowed many wealthy peasants to learn the basics of reading and writing.
The crafts and minor industries in the cities were equally much higher developed in China than in Europe. Textile industry first provided an extra income to the peasant families, but later developed to a separate industrial branch with factory workers, especially in Songjiang 松江 near later Shanghai, Suzhou 蘇州/Jiangsu and Hangzhou 杭州/Zhejiang - a city famous for its silk production. Tea plantations in Zhejiang and Fujian did not only deliver their products to all places in China, but produced also goods for export to Europe, especially England. Another item exported to Europe was chinaware or porcelain (ciqi 磁器; often called "earthenware" taoqi 陶器), produced in the state-owned kilns in Jingdezhen 景德鎮/Jiangxi, or in private porcelain producing cities like Lijiang 醴降/Hunan, Zibo 淄博/Shandong, or Dehua 德化/Fujian. Paper, hempen cloth, lacquerware, and metal objects also belonged to the early industrially produced commodities. Wuhu 蕪湖/Jiangsu was a center of steel production.
"International" trade exists since we find states in human history, but China never had a system of import taxes, or customs convention. Trade and traffic with foreign countries originated already in the period of Warring States between China and the Inner Asian nomad tribes, later with the Korean kingdoms, with Japan, South East Asia, Tibet and India. The high export rates of tea, porcelain and other agricultural or industrial products to Europe were rewarded with a very positive balance of payments (if this modern term may be allowed to use) - Chinese merchants and the state were payed with silver coins made of silver from the Americas.
To provide the whole country with good and items needed, an intense trade system was necessary since the Sui Dynasty, when the Great Imperial Canal was dug. In China, the waterways had always had a much higher importantce for trade and commerce than the land routes or the sea traffic along the coast. Until today, the canal system in the Yangtze area serves as the main transport medium. Since the days of Tang Dynasty, merchants and traders took over the resonsibility to transport not only wares of private origin, but also commodities that were subject to state monopoly, like salt and liquor.
Last but not least, we can see that from the Manchu conquest of whole China until the First Opium War, there were almost no military conflicts with foreign powers or inside the empire - a long period of peace.
Economical Changes: Jiaqing, Daoguang
While the first half of the 18th century was a time of prosperity, corruption and favoritism at the end of the century helped to create hopless situations for peasants in many areas. The White Lotus Sect (Bailian Jiao 白蓮教) was revived and helped to launch peasant uprisings in territories where the mismanagement of local magnates and magistrates had neglected the maintenance of dikes and waterways and had lead to flood disasters. Other peasant uprisings followed a secret society named Triad Sect (Sanhe Hui 三合會). The suffering of peasantry in many areas was worsened by the demographic increase of population during the 18th century. The economical and technical standards of the 18th China were quite high, but they did not fit the needs and demands of an increasing population. Qing China did not make use of paper money but instead relied on copper and silver coins. When the import of silver decreased - or rather the export of silver increased - at the begin of 19th century, the small copper coin ("cash") suffered depreciation: a fatal situation for the lower classes of society. Corruption, favoritism, and nepotism within the Chinese officialdom has two sources. The first can be seen in the exaggerated centralism of Qing administration. Governmental posts in the territorial administration were occupied by officials that came not from actual province, but the magistrates had to rely on the help of local secretaries and the local gentry and therewith had personal relations to these people. The second reason for the spoliation and nepotism mentality is the fact that - after passing the difficult state examinations and obtaining a post as local governor - the newly posted official had to reward his sponsors and his family as long as he was sitting on his post. Additionally, the daily flood of paperwork in a centralized bureaucracy lead to severe cautiousness and inflexibility of the officialdom. Paralysed by administratorial instructions and controled by censorate inspectors, local officials were unable to cope with new challenges in a changing enviroment. The state itself run into financial crisis after decades of prosperity, and the requirements for financial stability within an unstable economy were to high at the begin of 19th century. While the small states of Europe could develop an industrial and capitalist economy, the agronomical background and the loss of monetary investment could not help China in her backwardness that became so obvious when the agressive European merchants tried to enter the Chinese market.