Complex sentence: The replies to the circular he sent out and the subsequent correspondence amount to the largest single collection of primary source material on the subject ever assembled and the British Museum purchased the collection after his death and they are now in the British Library.
Split into simpler sentences: The replies to the circular he sent out and the subsequent correspondence amount to the largest single collection of primary source material on the subject ever assembled. The British Museum purchased the collection after his death and it is now in the British Library.