THE COiMBUSTlOA' OF OIlGAiMC MATTKR. 149 



A full grown man, therefore, gives off from his lungs, in the course of a 

 year, upwards of 100 lbs. of carbon in the form of carbonic acid. 



If the (juanlity of carbon thus evolved from the lungs be in proportion 

 to the weight of the animal, a cow or a horse ought to give off six tipies 

 as much as a man.* From indirect experiments, however, Boussin- 

 gault estimated the quantity of carbon actually lost in this way by a cow. 

 at 2200 grammes in 24 hours, and by ahorse at 2400 grammes. — [Ann. 

 de Chim. tide Phys., Ixxi., pp. 127 and 136.] These quantities are equal 

 to 6 or 7 times the amount of carbon given off from the lungs of a man. 



If we suppose each inhabitant of Great Britain, young and old, to ex- 

 pire only 80 lbs. of carbon in a year, the twenty millions would emit 

 seven hundred thousand tons ; and, allowing the cattle, sheep, and all 

 other animals, to give off twice as much more, the whole weight of 

 carbon returned to the air by respiration in this island would be about 

 two millions of tons, or the quantity abstracted from the atmosphere by 

 four millions of acres of cultivated land. 



Whence is all this carbon derived ? It is a portion of that which has 

 been conveyed into the stomach in the form of food. Suppose the car- 

 bon contained in the daily food of a full grown man to amount to one 

 pound — which is a large allowance — then it appears that, by the ordi- 

 nary processes of respiration, at least one-third of the carbon of his food 

 is daily returned into the air. 



In other animals the proportion returned may be different from what 

 it is in man, yet the life of all depends on the emission to a certain ex- 

 tent of the same gas.f And since all are sustained by the produce of 

 the soil, it is obvious that the process of animal respiration is one of 

 those methods by which it has been provided that a large portion of the 

 vegetable productions of the globe should be almost immediately re- 

 solved into the simpler forms of matter from which it was originally 

 compounded, and again sent up into the air to minister to the wants of 

 new races. 



II. ON THE PRODUCTION OF CARBONIC ACID BY COMBUSTION. 



Another important source of carbonic acid is familiar to us in the re • 

 suits of artificial combustion. 



In the previous lecture I have shown how, by the action of the sun's 

 rays upon the leaf, the carbonic acid absorbed from the atmosphere is 

 deprived of its oxygen, and its carbon afterwards united to the elements 

 of water for the ])roduction of woody fibre. During the process of com- 

 bustion, this labour of the living leaf is undone — the carbon is made to 

 combine anew with the oxygen of the atmosphere, and the vegetable 

 matter is resolved again into carbonic acid and water. 



Thus, when wood (woody fibre) is burned in the air, oxygen disap- 

 pears, and carbonic acid and watery vapour are alone produced. The 

 theory of this change is simple. 



* Estimating the ordinary weight of a man at 150, and of a cow at 800 to 900 Ibs.-'See 

 Sprengel, Lehre vom Dilnger, p. 208. 



* That the proportion mtcst be less in the larger animals is certain, since the daily food of 

 a cow may be stated generally as equivalent to 25 lbs. of hay, containing upwards of 10 lbs. of 

 carbon. If one-third of this were given off from the lungs, the quantity of carbon (SJ lbs.) 

 evolved would be ten times greater than was indicated by the experiments of BoussingauU, 

 and nearly double of what Uie weight of a cow, compared with Uiatof a man, requires. 



7* 



