The X-Patents are all the patents issued by the United States Patent and Trademark Office from July 1790 (when the first U.S. patent was issued), to July 1836. The actual number is unknown, but the best estimate is 9,957. The records were burned in a fire, in December 1836, while in temporary storage. No copies or rosters were maintained by the government at the time, leaving only the inventors' copies to reconstruct the collection.
The USPTO and its earliest days
On July 31, 1790, inventor Samuel Hopkins of Pittsford, Vermont became the first person to be issued a patent in the United States. His patented invention was an improvement in the "making of Pot Ash by a new apparatus & process." The earliest patent law required that a working model of each invention be produced in miniature.
The Patent Law was revised for the first time in 1793. It adopted a simple registration system where a patent would be granted for a $30 fee. The Patent Board was replaced by a clerk in the Department of State. James Madison, Secretary of State, created a separate Patent Office within the State Department and he appointed Dr. William Thornton as its first superintendent in May 1802.
The Patent Office was the only major government building to survive the British invasion of Washington, D.C., during the War of 1812. This is credited to Dr. William Thornton who was building a musical instrument in the same building. He persuaded British officers that they would be destroying the shared intellectual record of mankind if the patents were burned. 
The 1836 fire
The Patent Office 1836 fire occurred on December 15 when the patents were in temporary storage while a new (more fireproof) facility was being built.
Recovery of the X-Patents
Up until five months prior to the fire, U.S. patents had not been numbered, and were identified by titles and dates. The first patent denoted with the serial numbering system still in use today was issued on July 13, 1836, and was given the number 1. The recovered patents are also numbered from 1, but these numbers have an "X" added to them. The X is generally added to the end of the number except for the first patent which has the X in the beginning of the number. Therefore, they are called X-Patents.
When an earlier patent was recovered and re-issued, the USPTO sometimes gave it a fractional number (e.g. 2960½X, issued on June 2, 1818; 8736¾X, issued on March 27, 1835) to preserve the correct sequence. Most, but not all, fractional patents are X-Patents.
The latest X-Patents were recovered in 2004 from the Dartmouth College archives. Of the 14 found, 10 were granted to Samuel Morey including the first known patent for an internal combustion engine.
A list of some X-Patents
|X1||Potash production||Samuel Hopkins||July 31, 1790||U.S. Patent X1||First U.S. patent|
|X72||Cotton Gin||Eli Whitney||March 14, 1794||U.S. Patent X72||Revolutionized cotton farming and textiles industry|
|X4378||Gas Or Vapor Engine||Samuel Morey||April 1, 1826||U.S. Patent X4378|
|X9000||Grate||N. Winslow||July 2, 1836||U.S. Patent X9000|
|X9430||Improvement in Fire-Arms||Samuel Colt||February 25, 1836||U.S. Patent X9430||A key patent in revolver history|
|X9899||Brick Machine||C. Waterman||July 2, 1836||U.S. Patent X9899|
|X9894||Plow Moldboard||I. Snider||July 2, 1836||U.S. Patent X9894|
|X9893||Lock||A. Roff||July 2, 1836||U.S. Patent X9893|
|X9890||Thrashing Mach.||A. Parson||July 2, 1836||U.S. Patent X9890|
|X9889||Cook Stove||W. Parmalee||July 2, 1836||U.S. Patent X9889|
|X9887||Plow||T. Miller||July 2, 1836||U.S. Patent X9887|
|X9886||Dressing Stares||C. McGregory||July 2, 1836||U.S. Patent X9886|
|X9885||Cotton Gin||J. McCreight||July 2, 1836||U.S. Patent X9885|
|X9884||Door Lock||J. Mo Clory||July 2, 1836||U.S. Patent X9884|
|X9882||Clover Huller||W. Loomis||July 2, 1836||U.S. Patent X9882|
|X9879||Feather Dresser||F.P.Knowlton||July 2, 1836||U.S. Patent X9879|
|X9878||Platform Balance||J. Horton||July 2, 1836||U.S. Patent X9878|
|X9877||Nail Extractor||R. Haynes||July 2, 1836||U.S. Patent X9877|
|X9876||Cotton Press||H.G. Guyon||July 2, 1836||U.S. Patent X9876|
|X9875||Cook Stove||C. Granger||July 2, 1836||U.S. Patent X9875|
- "Great Patent Fire of 1836". The United States Patent and Trademark Office. Retrieved November 23, 2007.
- Riordan, Teresa (August 9, 2004). "Lawyers Unearth Early Patents". New York Times.
- "Issue Years and Patent Numbers".
- Riordan, Teresa (August 9, 2004). "Lawyers Unearth Early Patents". New York Times. Archived from the original on May 9, 2013. Retrieved July 18, 2009.