CARBONIC ACID. 373 



cended ; in general, the proportion is very uniform. A pellicle is 

 formed on lime water, by exposure to the air ; it contains T | carbo- 

 nic acid, as formerly stated ; according to Dalton, T T7r , or even less. 



(h.) Although produced in enormous quantities by respiration, 

 combustion, and other processes, it is scarcely found to exist in great- 

 er proportion in large towns than in the country ; doubtless the winds 

 prevent its accumulation. At sea, however, only two leagues from 

 Dieppe, there was so little that it scarcely affected barytic water.* 



(i.) Caustic alkalies absorb carbonic acid gas entirely, and thus 

 separate it from other gases. 



(/.) Vegetation appears to be the grand means of preserving the 

 purity of the atmosphere ; it decomposes the carbonic acid, absorbs 

 its carbon for food, and lets loose its oxygen. 



It is true that vegetables emit carbonic acid in the night, but in 

 smaller quantity than that which they decompose in the day.f 



(k.) Carbonic acid is visible in the sunshine, as it descends into a 

 vessel of common air, because, on account of its great weight, it produ- 

 ces unequal refraction in the light, and thus creates a disturbed image. 



6. RESPIRATION. 



(a.) About 8 or 8J per cent, of carbonic acid is thrown from the 

 human lungs in respiration, at every expiration, and only 10 per 

 cent, when the contact is rendered almost as frequently as possible ; 

 a similar result happens with the whole animal creation. 



(b.) About 11 oz. Troy, of carbon, are thus daily detached from 

 the blood, and of course more than twice the weight of a living man 

 in a year. 



(c.) Thus one great office of respiration is, the decarbonization of 

 the blood. 



(d.) The production of animal heat is also intimately connected 

 with this process ; venous blood becomes arterial in the lungs, and 

 there acquires its florid color, and emits its excess of carbon, and its 

 capacity for heat, according to the experiments of Dr. Crawford, J is 

 enlarged from .892, which expresses the capacity of venous blood, 

 to .1030 ; thus the heat that would be evolved from the union of the 

 carbon with the oxygen, is absorbed, and again given out when the 

 arterial blood becomes venous, that is, all over the body.|| 



(e.) There can be no doubt that animal heat is connected also with 

 the nervous power, with secretion, and perhaps with other vital agencies. 



* Ann. Phil. N. S. VI, p. 75. t See Thomson's Chemistry. 



t The experiments of Dr. J. Davy, do not appear to have set aside those of Dr. 

 Crawford. 



|| In a note on respiration, in Parkes' Chem. Chat, the following facts are stated. 

 The human heart gives 100,000 strokes in 24 hours, 4000 strokes in an hour, and 

 66 or 67 in a minute, and 350 pounds of blood pass through it in that time ; 25 

 pounds is the whole amount in the body of a common sized man ; this passes through 

 the heart 14 times in an hour. The aorta of a whale is one foot in diameter, and 10 

 or 15 gallons of blood (half a barrel,) are sent out at every stroke with vast force. 



