§ 21. It has been considered that all danger from interments in towns would be obviated if no burials were allowed except at a depth of five feet. But bodies buried much deeper are found to decay; and so certain as a body has wasted or disappeared is the fact that a deleterious gas has escaped. In the towns where the grave-yards and streets are paved, the morbific matter must be diffused more widely through the subsoil, and escape with the drainage. If the interments be so deep as to impede escapes at the surface, there is only the greater danger of escape by deep drainage and the pollution of springs. Dr. Reid detected the escape of deleterious miasma from graves of more than 20 feet deep. He states— In some churchyards I have noticed the ground to be absolutely saturated with carbonic acid gas, so that whenever a deep grave was dug it was filled in some hours afterwards with such an amount of carbonic acid gas that the workmen could not descend without danger. Deaths have, indeed, occurred occasionally in some churchyards from this cause, and in a series of experiments made in one of the churchyards at Manchester, where deep graves are made, each capable of receiving from 20 to 30 bodies, I found in general that a grave covered on the top at night was more or less loaded with carbonic acid in the morning, and that it was essential, accordingly, to ventilate these grave-pits before it was safe to descend. This I effected on some occasions by means of a small chauffer placed at the top, and at one end of the grave a tube or hose being let down from it to the bottom of the grave. The fire was sustained by the admission of a small portion of fresh air at the top, and the air from the bottom of the grave was gradually removed as the upper stratum was heated by the fire around which it was conveyed; and when it had been once emptied in this manner a small fire was found sufficient to sustain a perpetual renewal of air, and prevent the men at work in the grave-pits from being subject to the extreme oppression to which they are otherwise liable, even when there may be no immediate danger. A mechanical power might be used for the same purpose; and chemical agents, as a quantity of newly slaked lime, are frequently employed, as they absorb the carbonic acid. From different circumstances that have since occurred, it appears to me probable that numerous examples of strata or superficial soil containing carbonic acid may be more frequently met with than is generally suspected, and that while in churchyards the presence of large quantities of carbonic acid may be frequently anticipated, its presence must not always be attributed solely to the result of the decomposition of the human body.