If thou hadst _a sorrow_ of thine own, the brook might tell thee
of it.--HAWTHORNE

In the first sentence, instead of the general abstract notion of
delight, which cannot be singular or plural, _a delight_ means one
thing delightful, and implies others having the same quality.

So _a sorrow_ means one cause of sorrow, implying that there are
other things that bring sorrow.

[Sidenote: _Become pure class nouns._]

NOTE.--Some abstract nouns become common class nouns with the
indefinite article, referring simply to persons; thus,--

If the poet of the "Rape of the Lock" be not _a wit_, who
deserves to be called so?--THACKERAY.

He had a little brother in London with him at this time,--as
great _a beauty_, as great a dandy, as great a villain.--_Id._

_A youth_ to fortune and to fame unknown.--GRAY.

[Sidenote: _Changes material to class nouns._]

191. _An_ or _a_ before a material noun indicates the change to a
class noun, meaning one kind or a detached portion; as,--

They that dwell up in the steeple,...
Feel a glory in so rolling