COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN 881 



been also made with great exactitude with large animals, such as men, 

 bulls, sheep, &c. By means of enormous hermetically closed bell 

 receivers and the analysis of the gases evolved during respiration it was 

 found that a man expels about 900 grams (more than two pounds) of 

 carbonic anhydride per diem, and absorbs during this time 700 grams of 

 oxygen. 1 It must be remarked that the carbonic anhydride of the air 

 constitutes the fundamental food of plants (Chapters III., V., and VIII.) 

 Carbonic anhydride in a state of combination with a variety of other 

 substances is perhaps even more widely distributed in nature than in a 

 free state. Some of these substances are very stable and form a large 

 portion of the earth's crust. For instance, limestones, calcium carbonate, 

 CaC0 3 , were formed as precipitates in the seas existing previously on 

 the earth ; this is proved by their stratified structure and the number 

 of remains of sea animals which they -frequently contain. Chalk, 

 lithographic stone, limestone, marls (a mixture of limestone and clay), 

 and many other rocks are examples of such sedimentary formations. 



1 The quantity of carbonic acid gas exhaled by a man during the twenty- four hours 

 is not evenly produced; during the night more oxygen is taken in than during the 

 clay (by night, in twelve hours, about 450 grams), and more carbonic anhydride is sepa- 

 rated by day than during night-time and repose ; thus, of the 900 grams produced 

 during the twenty-four hours about 375 are given out during the night and 525 by day. 

 This depends on the formation of carbonic anhydride during the work performed by the 

 man in the day. Every movement is the result of some change of matter, for 

 force cannot be self-created (in accordance with the law of the conservation of energy). 

 Proportionally to the amount of carbon consumed an amount of energy is stored up in 

 the organism and is consumed in the various movements performed by animals. This 

 is proved by the fact that during work a man exhales 525 grams of carbonic anhydride 

 in twelve hours instead of 875, absorbing the same amount of oxygen as before. After 

 a working day a man exhales by night almost the same amount of carbonic anhydride 

 as after a day of rest, so that during a total twenty-four hours a man exhales about 

 900 grams of carbonic anhydride and absorbs about 980 grams of oxygen. There- 

 fore during work the change of matter increases. The carbon expended on the work 

 is obtained from the food ; on this account the food of animals ought certainly to 

 contain carbonaceous substances capable of dissolving under the action of the digestive 

 fluids, and of passing into the blood, or, in other words, capable of being digested. Such 

 food for man and all other animals is formed of vegetable matter, or of parts of other 

 animals. The latter in every case obtain their carbonaceous matter from plants, in 

 which it is formed by the separation of the carbon from the carbonic anhydride taken up 

 during the day by the respiration of the plants. The volume of the oxygen exhaled by 

 plants is almost equal to the volume of the carbonic anhydride absorbed ; that is to say, 

 nearly all the oxygen entering into the plant in the form of carbonic anhydride is libe- 

 rated in a free state, whilst the carbon from the carbonic anhydride remains in the plant. 

 At the same time the plant absorbs moisture by its leaves and roots. By a, process which 

 is unknown to us, this absorbed moisture and the carbon obtained from the carbonic an- 

 hydride enter into the composition of the plants in the form of so-called carbohydrates, 

 composing the greater part of the vegetable tissues, starch and cellulose of the com- 

 position CflHjoOs being representatives of them. They may be considered like all carbo- 

 hydrates as compounds of carbon and" water, 60 + 5H 2 O. In this wa,y a circulation of 

 the carbon goes on in nature by means of vegetable and animal organisms, in which 

 changes the principal factor is the carbonic anhydride of the air. 



