CAISSON DISEASE, a group of morbid changes met with in caisson workers and divers in diving dress. In order to exclude water, air pressure within a caisson used for subaqueous works must be increased by one atmosphere, or 15 lb. per sq. in. for every 331ft. that the caisson is submerged below the surface. Hence at a depth of ioo ft. a worker in a caisson, or a diver in a diving-dress, must be subjected to a pressure of 6o lb. per sq. in. A similar condition obtains when caissons are used on land for driving tunnels. Exposure to such pressures is apt to be fol lowed by symptoms of a very varied character, including pains in the muscles and joints (the "bends"), deafness, embarrassed breathing, vomiting, paralysis ("divers' palsy"), fainting and sometimes even sudden death. At the St. Louis bridge, where a pressure was employed equal to 44 atmospheres, out of 600 workmen, 119 were affected and 14 died. Symptoms do not appear while the pressure is being raised nor so long as it is continued, but only after it has been removed ; they are due to effervescence of gases absorbed in the body-fluids during exposure to pressure. Experiment has proved that in animals exposed to compressed air nitrogen is dissolved in the fluids in accordance with Dalton's law, to the extent of roughly 1% for each atmosphere of pressure, and also that when the pressure is suddenly relieved the gas is liberated in bubbles throughout the body. Set free in the spinal cord, for instance, they may give rise to partial paralysis, in the labyrinth of the ear to auditory vertigo, or in the heart to stop page of the circulation. But if the pressure is relieved gradually they are not formed, because the gas comes out of solution slowly and is removed by the lungs. To prevent caisson disease, there fore, the decompression should be slow; Leonard Hill suggests it should be at a rate of not less than 20 minutes for each atmosphere of pressure. Good ventilation of the caisson is also of great importance and long shifts should be avoided, because by fatigue the circulatory and respiratory organs are rendered less able to eliminate the absorbed gas and because a high partial pressure of oxygen acts as a general protoplasmic poison. It has been found that liability to caisson disease increases with age. Hence healthy young men alone should be employed. If the slightest symptom occurs on decompression the sufferer should be re-compressed, and then decompressed more slowly.