4. =Certain Constituents of the Atmosphere= have an important purifying effect. Of these oxygen is by far the most important. By its means organic impurities become oxidised, and thus rendered harmless. It is probable that much of this oxidation is effected by means of ozone—a peculiarly active and concentrated form of oxygen. A large part of this ozone is probably produced during thunderstorms and similar electrical disturbances of the atmosphere. The ammonia and organic impurities in air become changed into nitrites and nitrates—chiefly of ammonium—and being washed down by rain, form an important part of the food of plants. 5. =For Chemical Measures= of purification of the atmosphere see page 324. CHAPTER XX. GENERAL PRINCIPLES OF VENTILATION. =The Amount of Air required.=—Ventilation is chiefly concerned with the removal of the products of respiration, just as sewage is chiefly concerned with the removal of the solid and liquid excreta. In a less degree it is required for removing the impurities produced by the burning of gas, candles, and lamps. The main problem, however, is the removal of the respiratory products. The amount of carbonic acid in air is usually fairly proportional to that of the other respiratory products. It may therefore be taken as a measure of the impurity of the air. There are, however, certain fallacies in this test. In soda water manufactory, for instance, there would be a comparatively harmless excess of carbonic acid. In dirty rooms, and in hospitals and other institutions where rooms are not vacated for a considerable period, the amount of organic matter present is often in excess of what would have been anticipated, judging by an estimation of the carbonic acid. This is strikingly shown by some valuable researches at Dundee, which are summarised in the following table. If we take the average amount (in excess of outside air) of carbonic acid, organic matter, and micro-organisms respectively in houses of four or more rooms as unity, then in one or two-roomed houses or tenements we have as follows:— ┌──────────────────┬──────────────┬──────────┬──────────┐ │ │HOUSES OF FOUR│TWO-ROOMED│ONE-ROOMED│ │ │ ROOMS │ HOUSES. │ HOUSES. │ │ │ AND UPWARDS. │ │ │ ├──────────────────┼──────────────┼──────────┼──────────┤ │_Carbonic acid_ │ 1 │ 1·5 │ 2·0 │ │_Organic matter_ │ 1 │ 1·6 │ 4·4 │ │_Micro-organisms_ │ 1 │ 5·1 │ 6·7 │ └──────────────────┴──────────────┴──────────┴──────────┘