I found this on New Advent:
THE AUTHORITY OF "APOSTOLICAE CURAE"
The question has been raised whether the pronouncement of the
Bull "Apostolicae Curae" is or is not to be taken as an
infallible utterance of the
Holy See. But even if it were not it would not follow that it can be disregarded, and its eventual withdrawal confidently anticipated. What may be safely assumed is that it fixes the
belief and practice of the
Catholic Church irrevocably. This at least
Leo XIII must have meant to signify when in his letter to Cardinal Richard, of 5 November, 1896, he declared that his "intention had been to pass a final judgment and settle (the question) forever" (
absolute judicare et penitus dirimere), and that "
Catholics were bound to receive (the judgment) with the fullest obedience as
perpetuo firmam, ratam, irrevocabilem".
Still, as a matter of speculative interest, it may be asked whether the definition is strictly
infallible, and the answer may be stated shortly thus. It belongs to a class of
ex cathedral utterances for which
infallibility is claimed on the ground, not indeed, of the terms of the
Vatican definition, but of the constant practice of the
Holy See, the consentient teaching of the
theologians, as well as of the clearest deductions from the principles of
faith.
To understand what is meant it is
necessary to bear in mind the distinction between a
dogma and a
dogmatic fact, the former being a
doctrine of
revelation, the latter a fact so intimately connected with a revealed
doctrine that it would be impossible without inconsistency to assert the former and deny the latter. It may be urged that the
Vatican Council merely
defined that the
Pope when speaking
ex cathedra has "that
infallibility which the
divine Redeemer wished His
Church to have in defining
doctrine of
faith and
morals", without going on to define the range of
infallibility which
Our Lord wished His
Church to have. But it must be
remembered
Seems that Leo did indeed to intend this to be the final Catholic word on the issue. And that it must be accepted as doctrine by the whole church.