On 7 June 1753, King George II gave his formal assent to the Act of Parliament which established the British Museum. The British Museum Act 1753 also added two other libraries to the Sloane collection, namely the Cottonian Library, assembled by Sir Robert Cotton, dated back to Elizabethan times and the Harleian library, the collection of the Earls of Oxford.
The museum first opened to the public on 15 January 1759, in Montagu House, on the site of the current building. Its expansion over the following two and a half centuries was largely a result of expanding British colonization and has resulted in the creation of several branch institutions, the first being the British Museum of Natural History in South Kensington in 1881 (it is nowadays simply called the Natural History Museum, and is separate and independent).
Although today principally a museum of cultural art objects and antiquities, the British Museum was founded as a "universal museum". Its foundations lie in the will of the Irish-born British physician and naturalist Sir Hans Sloane (1660–1753). At that time, Sloane's collection consisted of around 71,000 objects of all kinds including some 40,000 printed books, 7,000 manuscripts, extensive natural history specimens including 337 volumes of dried plants, prints and drawings including those by Albrecht Dürer and antiquities from Sudan, Egypt, Greece, Rome, the Ancient Near and Far East and the Americas.
The British Museum is home to one of the world's finest numismatic collections, comprising about a million objects, including coins, medals, tokens and paper money. The library covers all levels of education, from casual visitors, schools, degree level and beyond. The museum's various libraries hold in excess of 350,000 books, journals and pamphlets covering all areas of the museum's collection. Also the general museum archives which date from its foundation in 1753 are overseen by this department; the individual departments have their own separate archives and libraries covering their various areas of responsibility, which can be consulted by the public on application. The Anthropology Library is especially large, with 120,000 volumes.
According to wikipedia