AIR AND LIFE. 155 



per 1,000, which is the normal proportion of carbonic acid, may rise in 

 a tolerably well ventilated room to 0.54 and 0.70, or to 2.4 in an ill ven- 

 tilated sick room, and reach to 3.2 m a lecture room, 7.2 in a school- 

 room, and even 21 in a stable in the Alps where men and beasts are 

 huddled together in winter, the chinks being stopped against the cold, 

 there occurs a limit which can not be passed ; if the ratio increases, men 

 and animals must soon die, and the experiment is over, the production 

 of carbonic acid having come to an end. When the composition of the 

 surrounding atmosphere is the same as that of the air which each of us 

 expires (over 4 per cent carbonic acid, and less than 16 per cent oxygen), 

 death must soon result, because there is too much carbonic acid in the 

 air to allow that in the system to escape, and not enough oxygen for 

 the needs of the body. More will be said on this point later on. It is 

 enough here to show how considerable the ratio of carbonic acid may 

 become in confined space, and how much greater are the variations in 

 carbonic acid than in oxygen or nitrogeu. 



The cause of these variations is obvious. They are in close relation 

 to the variations in the production of the gas under consideration, and 

 upon this matter information is abundant. 



Carbonic acid is produced in many ways; it has many sources. One 

 of them has been referred to — animals and mankind. Bipeds and 

 quadrupeds, in fact all animals, indeed, all living organisms, are sources 

 of carbonic acid. All beings, from mere yeast cells to the lords of 

 creation, breathe; all or nearly all take oxygen from the air and return 

 carbonic acid to it. It is a familiar fact that fermentation in most 

 cases — in the case of sweet substances particularly — is accompanied by 

 a considerable production of carbonic acid. In wine-producing countries 

 cases of asphyxia often occur m the cellars where fermentation is going 

 on, owing to the amount of carbonic acid produced. All higher organ- 

 isms, plants, and animals have the respiratory function, and one of the 

 acts of respiration is the elimination of carbonic acid through the lungs. 

 This unceasing production of carbonic acid by living organisms, whether 

 plants or animals, is very variable in its activity, even within the limits 

 of the same species and of the same individual. It is well known that 

 the male produces more than the female, the adult more than the very 

 young or the very old individual, the strong more than the weak, etc. It 

 is well known, also, that this production of carbonic acid is increased by 

 exercise, movement, light, and food, while it is decreased by rest, dark- 

 ness, inanition. On the average each man exhales 20 liters of this gas 

 per hour, and nearly 1 kilogram per diem (of twenty-four hours). The 

 production is more considerable in sheep, and a bull exhales between 

 7 and 8 kilograms during the same lapse of time. However, in order 

 to well appreciate the ratio of carbon dioxide exhalation, instead of 

 considering the whole amount produced by any individual, it is better 

 to refer this amount to the weight of the individual animal or person, 

 to ascertain the quantity evolved per kilogram of weight. Viewing the 



