524 THE AIR AND LIFE. 



series were it not tliat analysis shows their i)art in life to be almost as 

 important as the fundamental and essential elements. 



The chief of the accessory elements is carbonic acid. Very small 

 quantities of it exist in the air in proportions of 4 or 5 to 10,000 vol- 

 umes of air. Relatively it is a heavy gas aud distinctly unfavorable 

 to respiration or combustion, as Priestley discovered. Its proportioniu 

 the air is not fixed, but varies according to place and condition mnch 

 more than does that of the other gases. De Saussure discovered very 

 marked variations as long ago as 1827, varying between 3-15 and ir)-74 

 per 10,000. Boussiuganlt and Levy found a difference between Paris 

 and Andilly of o-19 in the town and 2*99 in tlie village. Roscoe and 

 McDougall found not so much difterence between Manchester and its 

 environs, but at Clermont-Ferrand M. Truchot gives figures of o-15 

 per 10,000 ; at Pny de Dome, of 2-03, and at Pic de Sancey, 1-72. These 

 examples are enough to show that the quantity of carbonic acid in the 

 air varies considerably aud that the air of high places and of the 

 country is much purer than in towns. A variation also is produced by 

 the seasons, the month, and the year, andDe Saussure notes an increase 

 of the gas at night and in cloudy weather. But these variations occur 

 in an irregular manner from day today, although they seem less marked 

 above the sea, the air being purer there, as on high mountains. Again, 

 these variations are much greater in places where the air does not cir- 

 cnlate freely and where organic or inorganic combustions are taking 

 place. This is not a matter of surprise when we remember that the air 

 we have this moment exhaled contains 100 times more carbonic acid 

 than the same air when we inhaled it some seconds ago. This being 

 the case, we have only to take a close room with one or more persons in 

 it, and in time we could note all the possible proportions of carbonic 

 acid were it not tliat the experiment is self limited. The normal i)io- 

 portion of carbonic acid, as stated by Peftenkoffer, is 0-04 or O-O.") ]>er 

 1,000. This may increase in an ill aired room to 0-54 or 0-70, and in an 

 ill ventilated sick room to 2-4. In a lecture room it may go np to .■>-2, 

 in a schoolroom to 7-2, and even to 21 in an Alpine stable, where men 

 and beasts huddle together in winter, stopi)ing the chinks against the 

 cold. But there soon comes a limit; the men or the animals die sooner 

 or later and the productiim of carbonic acid ceases. They die, killed 

 by excess of carbonic acid and by lack of oxygen. A place containing 

 more than 4 per cent of carbonic acid and less than IG jier cent of oxj'gen, 

 which are the pro])ortions in expired air, becomes rapidly fatal to life. 

 But we shall take np this point again when we come to speak of the 

 relations of carbonic acid to life. Our object here is to point out that 

 the i)roportion of carbonic acid increases largely iu a confined loiality 

 and that its proportions vary more than those of oxygen and nitrogen. 



The variations depend npon the rate of production of the gas in 

 qnestion, and on that point we know relatively a good deal. Carbonic 

 acid has many sources, one of which we have mentioned in passing. 



