There are many lurking dangers, ready to destroy the living, in the burial-grounds of the present day. The mephitic vapors increase in quantity as decomposition advances, and become far more poisonous than either arsenic or prussic acid, if these were uncombined in their natural state. These dangerous graveyard gases can spread to quite a distance, and therefore can communicate the most malignant maladies at all times. Dr. Ayr claims that they extend to a distance of a hundred meters; some authorities assert that they reach sometimes twice the distance. This occurs generally when the grave is air-tight above, and the surface layer of the cemetery soil is imporous. Then the gas escapes where it finds the least resistance,—at the sides,—and burrows along under the earth until it strikes a cavity, and bursts into it, or diffuses into the air. When the grave offers no resistance above, the gas enters the atmosphere directly. Burial-grounds best fitted for cemetery purposes should be feared most, for it is evident that dryness and porousness are qualities which, although conducive to the rapid decay of a body, very much facilitate the escape of gases. The danger is not obviated by deep burials. In that case the morbific matter is diffused through the subsoil. If the inhumations are 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 and wells. Dr. Reid detected the escape of deleterious miasma from graves more than twenty feet deep. The danger from inhaling graveyard gases is great. Ramazzini relates how an avaricious grave-digger, by the name of Pisto, met with instantaneous death on descending into a vault to steal the shoes of a corpse; he was found dead upon the body. Lancisius (De noxiis palud. effluv. II, Ep. 1, c. 2, p. 152) states that several grave-diggers died in a like manner after entering a newly opened vault, which had been set under water by an inundation of the Tiber, and in which the stagnant water had regenerated the virulent gases. Unger gives an account of a case similar to that of Haguenot, reported further on. A vault was reopened in a convent at Madrid, for the purpose of depositing therein a fresh corpse. When the grave-digger was about to descend into it, he fell down dead. Two other persons, who tried to save him, shared his fate. Fortunatus Licetus (De annull, antiquitt. c. 23) relates that three men, who went into a vault that was full of semi-decomposed bodies with the intention of robbing, lost their lives. When the bodies were extracted, they were found to be swollen and black. Th. Bartholini (Historiar. anat. rarior. C. IV, obs. 32, p. 296) made experiments in Denmark which confirm these reports concerning the lethal action of graveyard gases, and prove the especial danger from the gases of the dead long pent up in vaults. He affirms that these noxious gases often prove fatal, death being preceded by dizziness and fainting.