Since the first attempt by Klose,1in 1904, to render the urinary tract opaque to the roentgen ray, it has been generally accepted that the essential properties of an ideal medium are opacity to the roentgen ray and nontoxicity. At present, sodium iodid best combines these properties. The effort to obtain an iodid salt more slowly soluble and sufficiently inert to be injected directly into the tissues has led to the use of certain fats which have been saturated with iodin. Such preparations have been found to show excellent shadows with the roentgen ray, and have been employed by Forestier2for intratrac h e a l injections, and by Sicard3for subarachnoid injections, with no untoward effects. They likewise found that the toxicity of such a solution was exceedingly low and that it was absorbed only over a period of months when retained in the body. My purpose