Conversation so far:
When did William Thomson, 1st Baron Kelvin attend Cambridge?
William's father was able to make a generous provision for his favourite son's education and, in 1841, installed him, with extensive letters of introduction and ample accommodation, at Peterhouse, Cambridge.
What did William Thomson, 1st Baron Kelvin study at Peterhouse, Cambridge?
William Thomson, 1st Baron Kelvin took a lively interest in the classics, music, and literature; but the real love of his intellectual life was the pursuit of science.
Did William Thomson, 1st Baron Kelvin belong to any clubs at Peterhouse, Cambridge?
Did William Thomson, 1st Baron Kelvin have any other interests besides classics, music, and literature?
The study of mathematics, physics, and in particular, of electricity, had captivated William Thomson's imagination.
What did William Thomson, 1st Baron Kelvin do as a fellow of St. Peter's?
On gaining the fellowship, William Thomson spent some time in the laboratory of the celebrated Henri Victor Regnault, at Paris.
What did William Thomson, 1st Baron Kelvin do after his fellowship?
In 1846, William Thomson was appointed to the chair of natural philosophy in the University of Glasgow.
How long did William Thomson, 1st Baron Kelvin teach?
William Thomson, 1st Baron Kelvin refused to leave Glasgow, remaining professor of Natural Philosophy for over 50 years, until his eventual retirement from that post.

Question: What subject did he teach?
Self-contained rewrite: What subject did William Thomson, 1st Baron Kelvin teach?