  We construct the general Lagrangian for relativistic and heavy baryon chiral
perturbation theory with virtual photons to fourth order. We work out the
electromagnetic and strong isospin violating contributions to the nucleon
self-energy, the nucleon mass and the scalar form factor of the nucleon.
Electromagnetic effects for the shift to the Cheng-Dashen point can be as large
as 2 MeV. We also discuss the corrections to Weinberg's prediction for the
scattering length difference $a(\pi^0 p) - a(\pi^0 n)$ and show that they are
small.
