Abstract
ACCORDING to Hull's law (1) the radiopacity of a substance is in direct proportion to the third power of the atomic weight. It is for this reason that whenever radiopaque substances are employed only substances of high atomic weight are used, such as iodine, barium, bismuth, etc. A number of other elements with high atomic weights also have radiopacity, but another important requirement is that the substance must be of low toxicity for biological or medical application. Thorium is an element with a very high atomic weight (232.12) and it is obvious, therefore, that it possesses a very high degree of radiopacity. In general, the toxicity of thorium compounds is low. In spite of this, however, thorium compounds have not been extensively used as a diagnostic medium because they possess a very undesirable property, namely, all thorium compounds possess radio-activity. One of the thorium compounds is a colloidal suspension of thorium dioxide. This preparation gives excellent x-ray films but has been shown to p...
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