Following its establishment as capital city in 1782, Bangkok grew only slightly throughout the 18th and early 19th centuries. British diplomat John Crawfurd, visiting in 1822, estimated its population at no more than 50,000. As a result of Western medicine brought by missionaries as well as increased immigration from both within Siam and overseas, Bangkok's population gradually increased as the city modernized in the late 19th century. This growth became even more pronounced in the 1930s, following the discovery of antibiotics. Although family planning and birth control were introduced in the 1960s, the lowered birth rate was more than offset by increased migration from the provinces as economic expansion accelerated. Only in the 1990s have Bangkok's population growth rates decreased, following the national rate. Thailand had long since become highly centralized around the capital. In 1980, Bangkok's population was fifty-one times that of Hat Yai and Songkhla, the second-largest urban centre, making it the world's most prominent primate city.