Why is "an university" used, not "a university"?

Question

I happened to read J.S.Mill's writing, and found "an unversity" used in it, instead of "a university". I think "a university" is the right choice in the present-day English, but why did Mill use the phrase "an university", not "a university", in his text? Maybe, the English that was used when Mill was alive has a different rule regarding the article usage?

I'd appreciate it if you would answer my question.

What is special to an university on these subjects belongs chiefly, like the rest of Sits work, to the intellectual department.

An university exists for the purpose of laying open to each succeeding generation, as far as the conditions of the case admit, the accumulated treasure of the thoughts of mankind.

These sentences are quated from Inaugural address delivered to the University of St. Andrews by J.S.Mill.

Answer 1

https://archive.org/details/acriticalpronou00conggoog/page/n94/mode/2up

This would indicate that even in the 18th century, "university" was pronounced with a "y"

University, yu(1)-ne(1)-ve(2)r'se(1)-te(1) s.   
A school, where all the arts and faculties are taught and studied.

in a modern ipa that would be /ˌjɪu.nɪˈvɛɹ.sɪ.tɪ/ which is not far off a modern /ˌjuː.nɪˈvɜː.sə.ti/.

However the /y/ sound was thought to be a semivowel, and there seems to have been some variation in whether this triggered the shift from a to an.

There tends to be less consistency in spelling the further you go back. This I would treat as an oddity. It would be considered an error today.

Answer 2

As I'm sure you know, 'an' is used as the indefinite article when preceding a vowel sound that would make the transition from 'a' sound awkward. It is about the pronunciation, not the spelling. So, as pronunciations of certain words have shifted slightly over time, the choice of 'a' or 'an' has adjusted

In the time of Mills, the 19th century, 'university' was more likely to be pronounced 'eeu-niversity' than 'yoo-niversity' as it is today.

There are also a few exceptions. The most widely known one is word 'historic'. Some insist that this should be preceded by 'an'. Dictionaries tend to note that either is acceptable.