What was J.R.R. Tolkien’s first published scholarly work on a constructed language?

J.R.R. Tolkien’s first published scholarly work on a constructed language was A Secret Vice.

Tolkien wrote the essay in 1931 and presented it as a lecture to the Johnson Society at Pembroke College, Oxford. It was not published during his lifetime; the text appeared posthumously in 1983 in The Monsters and the Critics and Other Essays. The work examines the personal and artistic pleasure of inventing languages.

In the essay, Tolkien discusses how constructed languages can be designed for sound, structure, and aesthetic effect. He also gives examples from languages he created, including early forms related to his Elvish languages. The title refers to his description of language invention as a private enthusiasm.

A Secret Vice is sometimes confused with Tolkien’s later essays on real historical languages, such as “English and Welsh.” Those works address linguistic history and philology, while A Secret Vice focuses directly on the creative construction of languages.