This is from an anecdote found in St. Nicholas magazine, Vol. 5, No. 4, (February 1878)

Sir Isaac Newton had on his table a pile of papers upon which were written calculations that had taken him twenty years to make. One evening, he left the room for a few minutes, and when he came back he found that his little dog "Diamond" had overturned a candle and set fire to the precious papers, of which nothing was left but a heap of ashes.