182 NON-METALS AND THEIR COMBINATIONS. 



animal body, and receives in exchange for the oxygen a quantity of 

 carbon dioxide, produced by the union of a former supply of oxygen 

 with the carbon of the different organs to which the blood is supplied. 

 The air issuing from the lungs contains this carbon dioxide, in 

 quantity about 4 volumes in 100 of exhaled air, which is 100 times 

 more than contained in fresh air. 



Exhaled air is, moreover, contaminated by other substances than carbon 

 dioxide, such as ammonia, hydrocarbons, and most likely traces of other or- 

 ganic bodies, the true nature of which has not been fully recognized, but which 

 seem to be directly poisonous. The bad effects experienced in breathing air 

 which has become contaminated by the exhalations from the lungs, are most 

 likely due to these unknown bodies. As we have as yet no methods of ascer- 

 taining the quantity of these poisonous substances present in exhaled air, the 

 determination of the amount of exhaled carbon dioxide present must serve as 

 an indicator of the fitness of an air for breathing purposes. As a general rule, 

 it may be stated that it is not advisable to breathe, for any length of time, air 

 containing more than 0.1 per cent, of exhaled carbon dioxide; in air contain- 

 ing 0.5 per cent, most persons are attacked by headache, still larger quantities 

 produce insensibility, and air containing 8 per cent, of carbon dioxide causes 

 death in a few minutes. 



As exhaled air contains from 3.5 to 4 per cent, of carbon dioxide, it is unfit 

 to be breathed again.. The total amount of carbon dioxide evolved by the 

 lungs and skin of a grown person amounts to about 0.7 cubic foot per hour. 

 Hence the necessity for a constant supply of fresh air by ventilation. This 

 becomes the more necessary where an additional quantity of carbon dioxide is 

 supplied by illuminating flames. 



Mentioned above are many processes by which carbon dioxide is 

 constantly produced in nature, and we might assume that the amount 

 of 0.04 per cent, of carbon dioxide contained in atmospheric air 

 would gradually increase. This, however, is not the case, because 

 plants, and more especially all their green parts, are capable of ab- 

 sorbing carbon dioxide from the air, while at the same time they 

 liberate oxygen. 



This process of vegetable respiration (if we may so call it), which 

 takes place under the influence of sunlight, is, consequently, the 

 reverse of that of animal respiration. The animal uses oxygen and 

 liberates carbon dioxide ; the plant consumes this carbon dioxide and 

 liberates oxygen. 



Carbon dioxide is an acid oxide, which combines with water, form- 

 ing carbonic acid : 



CO 2 + H 2 O = H 2 CO 3 . 



Carbonic acid, H 2 CO 3 , CO(OH) 2> is not known in a pure state, but 

 always diluted with much water, as in all the different natural waters. 



