Field shifts were derived from the mean value of the resonance line, defined as the field about which the first moment is zero.

Second moments of the spectra were computed by numerical integration. Corrections were applied for modulation broadening, apparatus background, and field shift.

Spectra were obtained over the temperature range of 77 -- 294 ` K. For the low-temperature measurements the sample was cooled by a cold nitrogen gas flow method similar to that of Andrew and Eades. The temperature was maintained to within about **f for the period of time required to make the measurement (usually about one hour). One sample, which had been exposed to the atmosphere after evacuation at 375 ` C, showed the presence of adsorbed water (about 0.3 wt%) as evidenced by a weak resonance line which was very narrow at room temperature and which disappeared, due to broadening, at low temperature. The data reported here are either from spectra from which the adsorbed water resonance could easily be eliminated or from spectra of samples evacuated and sealed off at 375 ` C which contain no adsorbed water.

The measured powder density of the **f used here was about **f, approximately one-third that of the crystal density (**f). Such a density corresponds to a paramagnetic ion density of about **f.

Spectra were obtained from a powdered sample having the shape of a right circular cylinder with a height to diameter ratio of 4: 1. The top of the sample was nearly flat and the bottom hemispherical. Spectra were also obtained from a sample in a spherical container which was made by blowing a bubble on the end of a capillary glass tube. The bubble was filled to the top and special precautions were taken to prevent any sample from remaining in the capillary. Spectra were also obtained from a third sample of **f which had been diluted to three times its original volume with powdered, anhydrous alundum (**f). This sample was contained in a cylindrical container similar to that described above.