  The paper is a colloquial-style discussion of invariants of algebraic
surfaces analogous to the Donaldson polynomials, arising from moduli spaces of
``jumping'' Yang--Mills instantons, or moduli spaces of jumping vector bundles.
The invariants have the following applications: (1) to the Van de Ven
conjecture that the Kodaira dimension is a diffeomorphism invariant; (2) to
proving that algebraic surfaces with $p_g > 0$ have a proper sublattice of
$H^2(X,\Z)$ invariant under diffeomorphism; (3) to proving the same result as
(2) for surfaces with $p_g = 0$, in particular the Barlow surface.
