History went up in smoke.
On Christmas Eve, 1851 there was a devastating fire in Washington D.C. that destroyed about two-thirds of some of the most important documents and items important to America's heritage. 35,000 of the 55,000 volumes in this library were destroyed including Thomas Jefferson's personal library, which was sold to the L-O-C in 1815.
While that blow to history was great the perseverance of America was greater. The library came back, and by the 20th century it had become the de facto national Library of the United States and one of the largest in the world. So large it is housed in three massive buildings, contains 17 million books, as well as millions of maps, photos, films etcetera.

Question:
What national treasure burned in a fire on Christmas Eve, 1851?