From recent chemical and microscopical examinations of the air of some crowded and filthy localities in the metropolis, it appears as a general result, that decomposing organic matter is always contained in such air,—the never-failing presence of animalcules testifying its existence, and their number and size indicating its amount. Imagine the state of the atmosphere in the dormitories of the Tooting children: in the sixty-eight cubic feet of breathing space of the inmates of the Taunton Workhouse; in the eighty cubic feet of the Kentish hop-pickers; in the four corners and centre of the five-family room. Conceive the state of the atmosphere in this room at night; all the members of the several families, collected; every breath of external air excluded; the windows, and perhaps even the chimney, carefully fastened up. This stagnant and poisoned air, breathed over and over again by every individual for seven or eight hours continuously; respiration, the special and admirable apparatus which nature has constructed for purifying the blood, thus made the very means of corrupting it. I have known from two to three cases of typhus produced nightly, for a fortnight together, in a room of this description, by sleeping in it for a single night! Can we wonder at the generation of typhus in such a room in _ordinary_ seasons! Can we wonder at the spread and the havoc of an epidemic in it in _epidemic_ seasons? * * * * * But besides the contamination of the air by external causes, it is conceived that the atmosphere itself undergoes natural changes which predispose it to the development and spread of epidemics. From time immemorial, the popular belief has been that such changes do take place, and that they manifest themselves by unmistakeable signs. Among such signs may be reckoned,—a disturbance of the regular and ordinary condition of the atmosphere; an inversion of the seasons—summer in winter, and winter in summer; long-continued drought succeeded by torrents of rain, causing rivers to overflow, and the seed to rot in the earth; cloud, mist, fog, favouring excessive dampness, under the influence of which spring up inordinate growths of the lower species of plants, producing mouldiness, and the blood-spots, and other coloured vegetation that adhere to houses, and household furniture, and wearing apparel, and personal ornaments, and the person itself; under which also, fostered by a steadily elevated temperature, spring into being and activity, myriads of the lower tribes of animals—locusts, caterpillars, flies,[7] frogs, covering the face of the earth, and devouring every green thing that the deluge of rain had left; and, as the sequence of these antecedent conditions, dearth and famine, closing the long series of the year’s calamities. Such, in all ages and countries, have been the recognized portents and precursors of a coming year of pestilence. Footnote 7: