  The paper studies the validity of Maxwell equation in the case for
coexistence of electromagnetic field and gravitational field. With the algebra
of quaternions, the Newton's law of gravitation is the same as that in
classical theory of gravitational field. Meanwhile the Maxwell equation is
identical with that in classical theory of electromagnetic field. And the
related conclusions can be spread to the case for coexistence of
electromagnetic field and gravitational field by the algebra of octonions. The
study claims that Maxwell equation keeps unchanged in the case for coexistence
of gravitational field and electromagnetic field, except for the direction of
displacement current.
