is of great importance when the only dependence is upon natural ventilation, for that is greatly facilitated by any increase of the extent of exposed surfaces, and of doors and windows. We should also keep in view that a like quantity of air will more readily traverse a large than a small space without producing inconvenient currents; and that the air in a large space requires less frequent renewal, and does not have to be kept in as rapid motion. Natural ventilation, which is uniform and almost insensible, must not be confounded with draughts and currents of air, with the injurious effects of which all are acquainted.
The rules as to the amount of space that should be allowed in connection with natural ventilation are various and indefinite. Aëration from this source can not always, however, be depended upon; and even the opening of windows on opposite sides of an apartment frequently fails to produce the changes of air that are needed. General Morin, who has distinguished himself as an apostle of ventilation, and who made numerous experiments bearing upon the subject, has given the following estimates of the volume of air that should be withdrawn and introduced every hour, for each person, in public institutions of different kinds: Children's schools, twelve to fifteen cubic metres; schools for adults, twenty-five to thirty cubic metres; amphitheatres, thirty cubic metres; assembly-halls and long-continued meetings, sixty cubic metres; play-houses, forty cubic metres; barracks, thirty cubic metres during the day, forty to fifty cubic metres at night; hospitals for the ordinary sick, sixty to seventy cubic metres; hospitals for the wounded and for women in childbirth, one hundred cubic metres; the same in times of epidemic, one hundred and fifty cubic metres; prisons, fifty cubic metres; stables, one hundred and eighty to two hundred cubic metres. These numbers certainly represent the maximum of reasonable demands; and M. Bouchardat thinks that they are exaggerated and not justified by clinical experience. Besides effecting the renewal of the air, ventilation also furnishes the means of obtaining a nearly constant temperature—in winter by means of the circulation of hot air through the house, in summer by air drawn from the cellar. The latter method is quite effective for securing an agreeable temperature in hot weather without much expense, whenever a sweet, dry cellar can be had. The cabinet of the Conservatoire des Arts et Métiers, in Paris, is kept cool in this way, the draught of air being promoted by gas-jets kept burning in the ventilating shafts; as is also M. Daville's laboratory at the Normal School, where the opening of a few squares in the glass-roof furnishes the required stimulus to the circulation. Similar principles have been adopted at the palace of the Corps Législatif. The subject of applying the artificial refrigeration of the air in colonial life in hot countries has been studied by M. Dessoliers, and elaborated by him with a number of ingenious devices, among which the storing of cold night-air for use during the day plays a part.