  In this paper, we consider the Hessian matrices $H_{\Gamma}$ of the complete
and complete bipartite graphs, and the special value of $\tilde H_{\Gamma}$ at
$x_{i}=1$ for all $x_{i}$. We compute the eigenvalues of $\tilde H_{\Gamma}$.
We show that one of them is positive and that the others are negative. In other
words, the metric with respect to the symmetric matrix $\tilde H_{\Gamma}$ is
Lorentzian. Hence those Hessian $\det (H_{\Gamma})$ are not identically zero.
As an application, we show the strong Lefschetz property for the Artinian
Gorenstein algebra associated to the graphic matroids of the complete and
complete bipartite graphs with at most five vertices.
