Emulsions of the insoluble salts of bismuth and barium have proved to be very satisfactory mediums for the roentgen examination of the gastro-intestinal tract. Emulsions, however, are not so satisfactory for similar examinations of the genito-urinary tract, in which it is essential to have a stable, homogeneous medium of as low a viscosity as possible. Nontoxicity and opacity to the roentgen ray are, of course, the prime essentials for such mediums. Nontoxic colloidal solutions of heavy metals, also even a few emulsions, such as that of silver iodid, have proved to be quite satisfactory. But on account of some of their physical properties, not to mention difficulty in their preparation, none of these mediums has proved to be ideal. In the spring of 1915, Burns published his first report of the use of thorium nitrate in pyelography. This solution has a number of advantages over the mediums in common use