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William Small (1734–1775) was a Scottish physician and a professor of natural philosophy at the College of William and Mary in Williamsburg, Virginia. There he became an influential mentor of Thomas Jefferson, who went on to be a leading proponent of the American Revolution, principal author of the Declaration of Independence, and the third President of the United States. From 1765, Small was an influential medical doctor in Birmingham, England, where he was a member of the Lunar Society.

Small was born in Carmyllie in Angus, Scotland, the son of a Presbyterian minister James Small and his wife Lillias Scott, and younger brother to Dr Robert Small. He attended Dundee Grammar School, and then Marischal College at the University of Aberdeen, where he received an MA in 1755.

In 1758, he was appointed Professor of Natural Philosophy at the College of William and Mary in Williamsburg, Virginia, then one of Britain's American colonies.

Small was Thomas Jefferson's professor at the College of William & Mary, and had an influence on the young Jefferson. Small introduced Jefferson to members of Virginia society who proved influential in Jefferson's life, including George Wythe, a leading jurist in the colonies and Francis Fauquier, the Governor of Virginia.

Recalling his years as a student, Jefferson described Small as:

a man profound in most of the useful branches of science, with a happy talent of communication, correct and gentlemanly manners, and a large and liberal mind... from his conversation I got my first views of the expansion of science and of the system of things in which we are placed.

In 1764, Small returned to Britain with a letter of introduction to Matthew Boulton from Benjamin Franklin. Through this connection with Franklin, Small helped form the Lunar Society, a club of scientists and industrialists.

In 1765, he received his MD degree and established a medical practice in Birmingham, where he shared a house with fellow physician John Ash, the chief campaigner for the Birmingham infirmary. Small was Boulton's physician and became a close friend of Erasmus Darwin, Thomas Day, James Keir, James Watt, Anna Seward, and others connected with the Lunar Society. Small was among the most popular members of the society and an active contributor to the society's debates.

Small helped to bring the Theatre Royal to Birmingham in 1774 and, together with Ash, was involved in planning and building Birmingham General Hospital, which was completed in 1779.

Small died in Birmingham on 25 February 1775 from malaria contracted during his stay in Virginia and was interred in St. Philip's churchyard in Birmingham.

The William Small Physical Laboratory, which houses the Physics Department at the College of William & Mary in Williamsburg, Virginia, is named in Small's honour.

• Hull, Gillian (1997). "William Small 1734-1775: No Publications, Much Influence". Journal of the Royal Society of Medicine. 92 (2): 102–105. doi:10.1177/014107689709000214. PMC 1296150. PMID 9068443.
• Walker, William T. "Professor William Small 1734-1775". William & Mary.
• Walcot, Patrick A Sketch of the Life of Dr William Small and his relationship with Matthew Boulton and James Watt. 2016

• Ganter, Herbert L. William Small, Jefferson's Beloved Teacher William and Mary Quarterly, 3rd Ser., Vol. 4, No. 4 (Oct., 1947), pp. 505–511