There are various methods of ascertaining the quality of the air in enclosed spaces, of which not the least useful is the information furnished by the _sense of smell_, on entering a room from the external air. Besides the evidence given by the senses, _chemical_ and _microscopical_ examination of the air gives important information, while the _thermometer_ and _hydrometer_ ascertain the temperature and degree of moisture. =Examination by the Senses.=—The dull grey haze hanging over a town, when it is viewed from a distance, indicates comparative impurity of its atmosphere, and the presence of a considerable amount of suspended matter, including smoke. The _smell_ of a stagnant atmosphere is a good preliminary guide to its condition. The fact that a room has been occupied for some time without efficient ventilation can be at once detected on entering a room from the external air. The sense of smell is extremely delicate; it has been estimated that the 3∕100,000,000 part of a grain of musk can be apprehended by it. But nothing is so soon dulled as the sense of smell. An atmosphere which did not appear to be unpleasant while remaining in a room, is intolerable when one returns to it after a few minutes in the open air. It is important not to confound the “closeness” perceived by the sense of smell, with the oppression due to the high temperature of a room. The two are easily distinguished (unless the two co-exist) by a reference to the thermometer, which ought always to be placed in rooms inhabited during the evening. The remedy for a close room is to allow free entry of fresh air, and _not_ allow the fire to go down, as is so commonly done, under the impression that the closeness is due to heat. De Chaumont has made many experiments, shewing how accurate is the information given by an acute sense of smell. Carbonic acid is destitute of odour, but as its amount is usually proportionate to that of the organic matter producing closeness, it may be taken as an index of the amount of impurity present in living rooms. De Chaumont found that the limit of smell is reached when carbonic acid amounts to 6 parts in 10,000 of air, or half as much again as in the external air. In the following extracts from his experiments, there was a close accordance between the evidence of his sense of smell and the amount of carbonic acid:— ┌────────────────────────────────────────────┐ │_At_ 14·8_ per_ 10,000 { _Extremely close │ │ { and unpleasant._ │ │ „ 10·90 „ _Extremely close._ │ │ „ 9·62 „ _Very close._ │ │ „ 9·21 „ _Close._ │ │ 8·43 „ _Not very foul._ │ │ „ 8·04 „ _Close._ │ │ „ 6·58 „ _Not very close._ │ │ „ 5·68 „ _Not close._ │ └────────────────────────────────────────────┘