To facilitate heats of wetting measurements, a metal calorimeter has been designed and constructed which employs a platinum resistance thermometer for temperature measurement and an evacuated dead space to minimize heat losses. Auxiliary circuits for the temperature bridge, and bath temperature control are shown. Initial measurements of the heats of solution of NaCl, both coarse and finely divided samples, indicate a precision of measurement of 0.08 percent. The surface energy of NaCl, calculated from these data, is 366 ergs/cm2.