148 HOW THE SUPPLY OF CARBONIC ACID IS RENEWED. 



A very short period, com pared even wiih the limits of authentic his- 

 tory, has yet elapsed since experiments began to be made on the true 

 constitution of the atmosphere ; we have no very trustworthy data, 

 therefore, on which to found a confident opinion in regard to the perma- 

 nence of the proportion of carbonic acid which it now contains. The 

 later observations of De Saussure do give a considerably lower estimate 

 ->f the quantity of this acid in the air than that which was deduced from 

 2ie results of the earlier experimenters; but the imperfection of the 

 <Tiode8 of analysis formerly adopted was too great, to justify us in rea- 

 soning rigorously from the inferences to which they led. We cannot 

 safely conclude from them that the proportion of carbon in the atmos- 

 phere has really diminished to any sensible extent during this limited 

 period; while the recorded identity of all the phenomena of vegetation 

 renders it probable that the proportion has not sensibly diminished even 

 within historic times. 



From what sources, then, is the supply of carbonic acid in the atmos- 

 phere kept up? — and if the proportion be permanent, by what compen- 

 sating processes is the quantity which is restored to the atmosphere 

 produced and regulated ? 



§ 3. How the supply of carbonic acid in the atmosphere is renewed 



and regulated. 

 On comparing, in a previous lecture, the quantity of rain which falls 

 with that of the watery vapour actually present in the air, we saw rea- 

 son to believe that even in a single year the same portion of water may 

 fall in rain or dew and ascend again in watery vapour several succes- 

 sive times. Is it so also with the carbon in the air ? Does that which 

 feeds the growing plant to-day, again mount up in the form of carbonic 

 acid at some future time, ready to minister to the sustenance of new 

 races, and to run again the same round of ever-varying change ? Such 

 is, indeed, the general history of the agency of the carbonic acid of the 

 atmosphere ; but when once it has been fixed in the plant it must pass 

 through many successive changes before it is again set free. The con- 

 ditions, also, under which it is restored to the atmosphere are so diver- 

 sified, and the agencies by which, in each case, it is liberated, are so 

 very distinct, as to require that the several modes by which the carbon 

 of plants is reconverted into carbonic acid and returned to the air, should 

 be made topics of separate consideration. 



I. ON THE PRODUCTION OF CARBONIC ACID BY RESPIRATION. 



The air we breathe when it is drawn into the lungs, contains 2x0*^ 

 of its bulk of carbonic acid ; when it returns again from the lungs, the 

 bulk of this gas amounts, on an average,* to Ysih of the whole ; or its 

 quantity is increased one hundred times. 



The actual bulk of the carbonic acid emitted from the lungs of a sin- 

 gle individual in 24 hours varies exceedingly ; it has been estimated 

 however, on an average, to contain upwards of five ounces of carbon. f 



* It varies in different individuals from 2 to 8 per cent, of the expired air. In animals it 

 varies also with tiie species. The air from the lungs of a cat contains from 5J to 7 per cent., 

 of a doa from Ah to 6^, of a rabbit from 4 to 6, and of a pigeon from 3 to 4 per cent, of the 

 whole bulk.— DuIong~ Annal. de Chim. etde P/it/s., third JSeriea, /., p. 455. 



t I>avy, and Allen, and Pepys, estimated the weight of carbon evolved in a day at upwards 

 of 11 ounces, a quantity which all writers have concurred in receiving with suspicion. 



