Invisible College is the term used for a small community of interacting scholars who often met face-to-face, exchanged ideas and encouraged each other. One group that has been described as a precursor group to the Royal Society of London consisted of a number of natural philosophers around Robert Boyle, such as Christopher Wren. It has been suggested that other members included prominent figures later closely concerned with the Royal Society; but several groups preceded the formation of the Royal Society, and who the other members of this one were is still debated by scholars. Background The concept of "invisible college" is mentioned in German Rosicrucian pamphlets in the early 17th century. Ben Jonson in England referenced the idea, related in meaning to Francis Bacon's House of Solomon, in a masque The Fortunate Isles and Their Union from 1624/5. The term accrued currency for the exchanges of correspondence within the Republic of Letters. Connection with Robert Boyle and the Royal Society Much has been made of an "invisible college" in London of the later 1640s. Revisionist history has undermined earlier narratives. Detailed evidence In letters in 1646 and 1647, Boyle refers to "our invisible college" or "our philosophical college". The society's common theme was to acquire knowledge through experimental investigation. Three dated letters are the basic documentary evidence: Boyle sent them to Isaac Marcombes (Boyle's former tutor and a Huguenot, who was then in Geneva), Francis Tallents who at that point was a fellow of Magdalene College, Cambridge, and London-based Samuel Hartlib. The Hartlib Circle were a far-reaching group of correspondents linked to Hartlib, an intelligencer. They included Sir Cheney Culpeper and Benjamin Worsley who were interested, among other matters, in alchemy. Worsley in 1646 was experimenting on saltpetre manufacture, and Charles Webster in the Oxford Dictionary of National Biography argues that he was the "prime mover" of the Invisible College at this point: a network with aims and views close to those of the Hartlib Circle with which it overlapped. Margery Purver concludes that the 1647 reference of "invisible college" was to the group around Hartlib concerned to lobby Parliament in favour of an "Office of Address" or centralised communication centre for the exchange of information. Maddison suggests that the "Invisible College" might have comprised Worsley, John Dury and others with Boyle, who were interested in profiting from science (and possibly involving George Starkey). Richard S. Westfall distinguishes Hartlib's "Comenian