The onset of solidification of 4 He in porous Vycor is identified from changes in transverse sound velocity. From observed overpressures required to initiate freezing (ranging from 18 bars at 1.3 K to 30.7 bars at 2.38 K), the interfacial energy between liquid and solid 4 He was estimated. The superfluid transition up to 35.5 bars was also observed.There has been considerable interest over the years in the behavior of 4 He in restricted geometries. Recently, observations 1 of the flow of liquid ^He in packed powders of various sizes have shown that, in small pores, 4 He does not solidify up to a pressure somewhat greater than the bulk melting pressure. Freezing of 4 He in alumina (500-A particle size) required overpressures of about 3.5 bars. Torsional-oscillator 2 and specific-heat 3 measurements have also been made in porous Vycor glass (a porous silica glass with a network of interconnected pores about 60 A in diameter) at pressures above the bulk melting curve. These indicated that the 4 He in the pores remained liquid for overpressures as large as 15 bars and, in fact, no indications of freezing were observed.In the experiments reported here, we made ultrasonic measurements of the transverse sound velocity in Vycor glass containing 4 He at pressures up to about 85 bars. The results indicate that the onset of solidification occurs at overpressures greater than about 18 bars. We also observed the X transition inside the Vycor at pressures up to 35.5 bars.The Vycor sample we used was a cylinder of diameter 0.36 cm and length 1.02 cm. The porosity was measured as -25%. The sample was mounted in a copper block and two 20-MHz LiNb0 3 shear transducers were bonded to the polished ends of the Vycor with high-viscosity silicone oil. The sample was inserted into the brass pressure cell (whose internal volume of 2.4 cm 3 was much larger than the 0.026 cm 3 pore volume of the sample). The transverse sound velocity in the Vycor was measured by using a pulse-echo overlap method with a resolution of 3 parts in 10 5 .The cryostat was a single-shot 3 He evaporation refrigerator capable of reaching 0.3 K. Temperatures were measured with a calibrated germanium resistance thermometer. Figure 1 shows representative velocity meas--urements. The points plotted with squares show the temperature dependence of the transverse sound velocity v t in the empty Vycor (normalized by the maximum value of v t ). The temperature dependence of v t is similar to that observed in fused silica. 4 However, both the transverse and longitudinal sound velocities in Vycor are considerably smaller than in fused silica because of the proosity of the Vycor. (t> t = 2.4xl0 5 cm/ sec, t>j = 3.7xl0 5 cm/sec in Vycor compared with ^ = 3.8xl0 5 cm/sec, f z = 6.0xl0 5 cm/sec in fused silica. It should also be noted that, at 20 MHz, the sound wavelength is 10" 2 cm, much larger than the pore size.) 1.001 1.000 0.999 o < o i-i o o > 0.987 0.986 0.985 0.984 0.983 h-0.982 0 12 3 4 TEMPERATURE (K)FIG. 1. Transverse velocity changes in empty and 4 He-filled Vycor as...