The dynamic nuclear polarization of hydrogen nuclei by the solid effect in single crystals of samarium doped lanthanum magnesium nitrate (Sm:LMN) has been studied theoretically and experimentally. The equations of evolution governing the dynamic nuclear polarization by the solid effect have been derived in detail using the spin temperature theory and the complete expression for the steady-state enhancement of the nuclear polarization has been calculated. For well-resolved solid effect transitions at microwave frequencies Ï ~ Ï _e ± Ï _n, the expression for the steady-state enhancement differs from the expression obtained by the rate equation approach by small terms which become zero at Ï ~ Ï _e ± Ï _n Experimental enhancements of the proton polarization were obtained for eight crystals at 9.2 GHz and liquid helium temperatures. The samarium concentration ranged from 0.1 percent to 1.1 percent as determined by X-ray fluorescence. A peak enhancement of 181 was measured for a 1.1 percent Sm:LMN crystal at 3.0^" K. The maximum enhancements extrapolated with the theory using the experimental data for peak enhancement versus microwave power and correcting for leakage, agree with the ideal enhancement (24O in this experiment) within experimental error for three of the crystals. The calculated satellite separation was within 6 percent of the measured separation for each of the enhancement curves and the peak positive and negative enhancements were equal for all but two of the crystals. The nuclear spin"lattice relaxation time was measured for one of the crystals between l.6^" K and 4.2^" K. To account for nuclear spin"lattice relaxation, spin diffusion theory in the rapid airrusion limit was incorporated into the results of the spin temperature theory of the solid effect. The experimental results indicate that the spin temperature theory is a quantitatively correct approach for the description of dynamic nuclear polarization by the solid effect for well"resolved solid effect transitions.