We report the results of a new method of measuring the capillary length for the dendritic crystal growth of non-faceted materials. This method uses a nearly spherical crystal held near unstable equilibrium in an oscillating temperature field. For the growth of ammonium chloride crystals from aqueous solution, previous published estimates of the capillary length varied by over a factor of 20. With this new method, we find the product of the diffusion constant and capillary length Dd(0) to be 0.78 +/- 0.07 mu m(3)/s, similar to that obtained for ammonium bromide crystals.