Small particles of metallic Sn were embedded in a BN matrix. The Mössbauer effect was measured around the melting point. The main results are : 1) At the melting point the resonance signal drops to an unmeasurable small value in a very limited temperature interval. Line broadening by diffusion could not be detected, confirming Packwood and Longworth. The observation of line broadening by Boyle et al. must have been due to impurities. 2) The resonance effect shows a pronounced hysteresis behavior due to undercooling. From the analysis of this phenomenon in conjunction with the Sn particle size an interphase energy between liquid and solid phase could be deduced. The energy (48 ergs cm-2) is in reasonable agreement with results by other techniques