Published $1851$.
In English:

Subject Matter

The first instance of the definition of the set.

The nature of infinity, and in particular Galileo's Paradox.

Historical Note

Paradoxien des Unendlichen was published in $1851$ after Bolzano's death by one of his students that he had befriended.

It lay unnoticed for some $20$ years.

Some sources state that it was published in $1850$, but this may be a mistake.