1. The functional residual air (defined as the lung volume at the end of normal expiration) has been determined in a series of normal individuals and in ten patients with lobar pneumonia at different stages of the disease.
2. The rate, depth, and minute volume of respirations were measured in the same individuals by a graphic method.
3. When appreciable cyanosis was present the oxygen content and capacity of the arterial blood were determined.
4. A constant relationship has been found to exist between the persistence and disappearance of symptoms (fever, accelerated heart rate, rapid and shallow breathing, cyanosis) and fluctuations of the functional residual air. When these symptoms persisted the functional residual air decreased; during their disappearance the volume of the functional residual air rose towards normal. The rise was detected soon after the crisis.
5. A close parallelism has been observed also between alterations in radiographic shadow, physical signs, and the volume of the functional residual air. The lung volume, measured at normal expiration, is diminished during the persistence of pathological signs in the lungs, and returns to normal as the pathological signs disappear. The average time required, in cases which recovered, for the functional residual air to become constant was 11 to 12 days, counting from the onset of the disease.