THE absence of radiographic density, or the negative density, of air and gases of all kinds causes them to cast distinctive shadows, familiar to all of us, on X-ray films. In addition to films of parts of the body which normally contain air, on films of any part of the abdomen the shadows of gas or air in the alimentary canal are always present. These vary in size from small bubbles and isolated pockets in the stomach and large intestine to wide inflation of the greater portion of the colon. On films of other parts of the body, however, the presence of shadows cast by air or gas is of rare occurrence. They are always found associated with injuries in which the skin is penetrated, with laceration of the underlying soft tissues, or in injuries which cause leakage of air from a damaged lung. The presence of air in wounds of these kinds is of minor importance, but the detection of gas caused by infections with anaërobic organisms is often of major clinical significance. Except during the World War, the subject of the radiographic detection of gas infections has received but little attention. To recall to the minds of radiologists and to point out to other medical practitioners the diagnostic importance of the shadows of air and gas on X-ray films, this investigation was undertaken (Figs. 1, 2). Radiolucent areas in the form of single or multiple oval or circular dark or black shadows have been found around the fractured ends of bones in compound fractures after the ends of the bone have been pulled back into the soft tissues. These shadows decrease rather rapidly in size and soon disappear. They are always limited to the region of the fractured ends of the bone, are usually of appreciable size, and are sharply defined. They are thought to be caused by bubbles of air in the soft tissues, air that was sucked into the wound when the ends of the bones were pulled back through the skin. Small quantities of air in the form of bubbles may be carried into the soft tissues in other types of penetrating, lacerating, or crushing injuries in which the skin is broken. The shadows of air have been found on films of both shotgun and pistol wounds. In the former, they are always located in proximity to the shot and never extend into the soft tissues in any direction from the actual location of the injury. In pistol-shot wounds, a few small bubbles of air may be found along the track of the bullet soon after the injury. In both instances the air is rather rapidly absorbed (Figs. 3, 4). Crushing and lacerating injuries, as of the hands and feet, when examined by means of X-ray films, will often show a number of bubbles of air in the soft tissues. These are of various sizes, with sharply marked borders. They are always limited to the region of the injury, never extending into uninjured tissue.