From The Collaborative International Dictionary of English v.0.48:
X \X\ ([e^]ks).
X, the twenty-fourth letter of the English alphabet, has
three sounds; a compound nonvocal sound (that of ks), as in
wax; a compound vocal sound (that of gz), as in example; and,
at the beginning of a word, a simple vocal sound (that of z),
as in xanthic. See Guide to Pronunciation, [sect][sect] 217,
270, 271.
[1913 Webster]
Note: The form and value of X are from the Latin X, which is
from the Greek [Chi], which in some Greek alphabets had
the value of ks, though in the one now in common use it
represents an aspirated sound of k.
[1913 Webster]