From The Collaborative International Dictionary of English v.0.48:
Ad- \Ad-\ [A Latin preposition, signifying to. See At.]
As a prefix ad- assumes the forms ac-, af-, ag-, al-, an-,
ap-, ar-, as-, at-, assimilating the d with the first letter
of the word to which ad- is prefixed. It remains unchanged
before vowels, and before d, h, j, m, v. Examples: adduce,
adhere, adjacent, admit, advent, accord, affect, aggregate,
allude, annex, appear, etc. It becomes ac- before qu, as in
acquiesce.
[1913 Webster]