In 1836, the US government inherited the fortune of an obscure British scientist who had never set foot in America.

It remains a mystery why James Smithson made the bequest, which has now grown into the largest museum and research complex in the world.

However, Heather Ewing has made a good stab at guessing why, in her book The Lost World of James Smithson (Bloomsbury, £20), which was launched last week at Smithson's alma mater, Pembroke College, Oxford.

The illegitimate son of the Duke of Northumberland, Smithson seems to have felt an outsider in England, not fitting in to the rigid class system. Inspired by his experiences at Oxford, where the new science of chemistry was gaining adherents (Percy Bysshe Shelley, at University College, was one of the students who became obsessed by it), he became the youngest member of the Royal Society of his day.

He left his fortune to his nephew Henry, and it was only because of Henry's early death with no children that the second clause of his will was activated, and the money went to establish a foundation in Washington "for the increase and diffusion of knowledge among men".

Ewing speculates that it was his illegitimacy, and desperation to fill the void of family history, that led him to make a gift to a land which he saw as unfettered by tradition, and representative of progress and a scientific future.