In the past, various substances have been used as opaque mediums in pyelography. Among such substances may be mentioned bismuth, the colloidal silver solutions (collargol, argyrol, electrargol, cargentos, etc.), and several preparations of silver iodid and thorium. Many of these substances, when retained in the renal pelvis, have a more or less irritating action on the kidneys, as has been demonstrated experimentally by Braasch and Mann, who injected the kidney pelves of sixty-seven dogs, and compared the effects of a variety of solutions. The results of their experiments show that the silver preparations act as foreign bodies, frequently causing multiple foci of necrosis, and that such focal necrosis may also occasionally occur when there is insufficient drainage from a kidney pelvis, even when bland fluids are used. Thorium nitrate in a 15 per cent, solution, as suggested by Burns, caused the least reaction of the various opaque mediums. This medium