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Nuclear resonance techniques involving free precession are examined, and, in particular, a convenient variation of Hahn s spin-echo method is described. This variation employs a combination of pulses of different intensity or duration ("90-degree" and "180-degree" pulses). Measurements of the transverse relaxation time T2 in Quids are often severely compromised by molecular diffusion. Hahn s analysis of the effect of diffusion is reformulated and extended, and a new scheme for measuring T2 is described which, as predicted by the extended theory, largely circumvents the diffusion effect. On the other hand, the free precession technique, applied in a different way, permits a direct measurement of the molecular self-diffusion ccnstant in suitable Quids. A measurement of the self-diffusion constant of water at 25'C is described which yields D=2.5(%0.3))&10 5 cm'/sec, in good agreement with previous determinations. An analysis of the effect of convection on free precession is also given. A null method for measuring the longitudinal relaxation time T&, based on the unequal-pulse technique, is described.

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The diffusion coefficient and the spin lattice relaxation time of protons in ordinary water have been measured in the temperature range 0-100'C using nuclear magnetic resonance free precession techniques. Unlike previous diffusion measurements, the present values describe the diR'usion of protons rather than foreign isotopes introduced as tracers. To within the experimental error the Stokes-Einstein relation adequately describes the relative temperature dependence of viscosity and diffusion, but above the vicinity of 40'C the spin lattice relaxation does not follow the viscosity in the predicted manner.

Nuclear magnetic resonance studies of natural xenon have been continued using fluid samples with improved purity. The Xe 129 spin-lattice relaxation time in the gas was found to be inversely proportional to the density: 1/7*1 = (5.0db0.5)X10 -6 p, where 7i is in sec and p in amagats. For the liquid in equilibrium with its vapor, the relaxation time throughout the temperature range 0° to -72°C is 1000±200 sec. In both the gas and the liquid the paramagnetic shift in the resonant value of the local field H at the nucleus, relative to its value for the isolated atom in the same external field, is directly proportional to the density and the external field: AH = + (4.22=L0.05)X10~7 pH 0 , where AH and H 0 are in G and p is in amagats. The above T\ and AH data agree well with a relationship derived by Torrey involving an analysis of the fluctuating magnetic field at a nucleus due to the rotation of a diatomic configuration. The derivation is based on Ramsey's theory of the chemical shift and the spin-rotational coupling which exist when two atoms are sufficiently close to interact. * Based on a