XXVIII
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Now you may infer that the earth contains many deadly elements from the mere fact that so many poisons grow of themselves without being sown; the soil no doubt contains seeds of evil as well as of good. Is it not the case that, earthquakes apart, in several places in Italy a pestilential steam is emitted through certain openings, which it is not safe for either man or beast to breathe? Even birds, if they meet it before it is neutralised by the purer breath of heaven, fall in mid-flight; their bodies become livid, and their jaws swell just as if they had been strangled. As long as this air is contained in the earth and escapes by a narrow opening, it has no greater power than to kill creatures that look down into, or voluntarily approach too near, it. But when for centuries darkness has brooded over it, and the gloom of the place has increased the infection, it becomes more dangerous through mere lapse of time; the more sluggish it is, all the more deadly does it become. Then when it has gained an exit it lets loose all that mischief conceived in the cold shades through endless ages of nether darkness, tainting with it the atmosphere of our realms of earth. The better is ever conquered by the worse. Even that purer air of heaven then changes to pestilential. Thence come sudden and continuous deaths, and portentous forms of disease that spring from unexampled causes. The disaster is long or short lived, according to the strength of the sources of infection. Nor does the plague cease until the freedom of heaven and the tossing of the winds have banished87 that fatal air.