This book expands upon ideas in the author's classic text Cosmical Magnetic Fields. This is the first book-length treatment of the magnetohydrodynamic theory of spontaneous magnetic discontinuities. Most continuous magnetic fields develop internal discontinuities as they relax to equilibrium. The formation of the tangential discontinuities are driven by the Maxwell stresses and provide sites for rapid reconnection and dissipation of the magnetic free energy, whichare the primary cause of the sun's x-ray emitting corona.