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A critical review of experimental technique for measuring high pressures has been made. The broad coverage includes discussions relating to (a) the establishment of a primary pressure scale using the free-piston gage, (b) the selection and precise measurement of identifiable phase changes as fixed pressure points, and (c) the use of interpolation and extrapolation techniques such as resistance gages, equations of state, and optical changes. The emphasis is on static pressure measurements above 10 kbar, but shock measurements are also considered for completeness. The pressure values to be associated with the fixed points have been analyzed in detail. Temperature measurement in the high pressure environment is also reviewed. The accuracy with which pressures can be measured has been carefully considered; the maximum accuracies now obtainable are considered to be of the order of 0.02 percent at 8 kbar, 0.25 percent at 25 kbar, 2 percent at 50 kbar, and 4 percent at 100 kbar.

A new polymorph of cesium, existing over the narrow pressure range from about 42.2 to 42.7 kilobars at room temperature, has been discovered. Its crystal structure is face-centered-cubic with a = 5.800 +/- 0.007 A at 42.5 kb and 27 degrees C. Cesium-II, which exists over the pressure range 23.7 to 42.2 kb, is also face-centered-cubic with a = 5.984 +/- 0.011 A at 41 kb and 27 degrees C. An improved resistance-pressure curve and x-ray compressibility measurements for Cs are also given.