N AND AIR 241 



especially of substances burning in it. 1 "' Hence it is necessary to have 

 regard to the purification of the air of dwellings. The renewal of air, 

 the replacing of respired by fresh air, is termed * ventilation,' 46 and the 



45 For this reason candles, lamps, and gas change the composition of air almost in 

 the same way as respiration. In the burning of 1 kilogram of stearin candles, 50 cubic 

 metres of air are changed as by respiration that is, 4 p.c. of carbonic acid will be 

 formed in this volume of air. The respiration of animals and exhalations from their skins, 

 and especially from the intestine and the excrements and the transformations taking place 

 in them, spoil the air to a still greater extent, because they introduce other volatile sub- 

 stances l>e>iile.^ carbonic anhydride into the air. At the same time that carbonic anhy- 

 dride is formed the amount of oxygen in the air decreases, and consequently the relative 

 amount of nitrogen increases, together with which there is noticed the appearance of mias- 

 mata which occur in but small quantity, but which are noticeable in passing from fresh 

 air into space full of such adulterated air. The researches of Schmidt and Leblanc and 

 others show that with 20'6 p.c. of oxygen (instead of 20'9 p.c.), when the diminution is due 

 to respiration, air already becomes noticeably heavy and unfit for respiration, and that the 

 heavy feeling experienced in such air increases with a lesser percentage of oxygen. It is 

 difficult to remain for a few minutes in air containing 17'2 p.c. of oxygen. These obser- 

 vations were chiefly obtained by observations on the air of different mines, at different 

 depths below the surface. The air of theatres and buildings full of people also proves to 

 contain less oxygen ; it was once found that at the end of a theatrical representation the 

 air at the stalls contained 20'75 p.c. of oxygen, whilst the air at the upper part of the theatre 

 contained only 20'36 p.c. The amount of carbonic anhydride in the air may be taken 

 as a measure of its purity (Pettenkofer). When it reaches 1 p.c. it is very difficult for 

 human beings to remain long in such air, and it is necessary to set up a vigorous ventilation 

 for the removal of the adulterated air. In order to keep the air in dwellings in a uniformly 

 good state, it is necessary to introduce at least 10 cubic metres of fresh air per hour per 

 person. We saw that a man exhales about five-twelfths cubic metres of carbonic anhy- 

 dride per day. Accurate observations have shown that air containing one-tenth p.c. of 

 exhaled carbonic anhydride (and consequently also a corresponding amount of the other 

 substances evolved together with it) is not yet felt as spoilt ; and therefore the five-twelfth 

 cubic metres of carbonic anhydride should be diluted with 420 cubic metres of fresh air if 

 it be desired to keep not more than one-tenth p.c. (by volume) of carbonic anhydride 

 in the air. Hence a man requires 420 cubic metres of air per day, or 18 cubic metres per 

 hour. With the introduction of only 10 cubic metres of fresh air per person, the amount 

 of carbonic anhydride may reach one-fifth p.c., and the air will not then be of the 

 required freshness. 



46 The ventilation of inhabited buildings is most necessary, and is even indispensable 

 in hospitals, schools, and similar buildings. In winter it is carried on by the so-called 

 calorifiers or stoves heating the air before it enters. The best kind of calorifiers in this 

 respect are those in which the fresh cold air is led through a series of channels heated by 

 the hot gases coming from a stove In ventilation, particularly during winter, care is taken 

 that the incoming air shall be moist, because in winter the amount of moisture in the 

 air is very small. Ventilation, besides introducing fresh air into a dwelling-place, must 

 also withdraw the air already spoilt by respiration and other causes that is, it is neces- 

 sary to construct channels for the escape of the bad air, besides those for the introduction 

 of fresh air. In ordinary dwelling-places, where not many people are congregated, the 

 ventilation is conducted by natural means, in the heating by fires, through crevices, 

 windows, and various orifices in walls, doors, and windows. In mines, factories, and works 

 ventilation is of the greatest importance. 



Animal vitality may still continue for a period of several minutes in air containing up 

 to 30 p.c. of carbonic anhydride, if the remaining 70 p.c. consist of ordinary air, but respi- 

 ration ceases after a certain time, and death may even ensue. The flame of a candle 

 is extinguished in an atmosphere containing from 5 to 6 p.c. of carbonic anhydride, 



VOL. I. R 



