52 RESPIRATION. 



28. [Carbonic acid gas, which extinguishes bodies in combus- 

 tion in the same way as azote, is formed by the combustion 

 oi' charcoal ; also, during the fermentation of wine, and of beer, 

 which makes it sparkling and frothy. 



-. "29. It is upon the action of this gas on the animal economy that 

 the asphyxia, produced by the vapor of charcoal, depends, as well 

 as the greater number of accidents of the same sort which occur 

 in mines, caves, wells, and vats wherein wine or beer is fer- 

 menting. -In a grotto near INaples, this gas is continuously disen- 

 gaged from the earth, and gives rise to phenomena, which, at first 

 sight, appear very singular, and excite the admiration of the 

 traveller ; when a man enters this cavern he experiences no in- 

 convenience in his respiration ; but a dog following him very soon 

 falls down in a state of asphyxia at his feet, and would soon ex- 

 pire, were he not speedily removed to the pure air. This arises 

 from the fact that the carbonic acid gas being much heavier than 

 the air, sinks down and forms upon the bottom of the cave a bed 

 or stratum of about two feet thick. Now, a dog that enters the 

 grotto is necessarily plunged over his head into this mephitic 

 gas, and must necessarily become asphyxiate, while a man who is 

 very much taller, only has the lower part of his body exposed to 

 the action of the carbonic acid, and breathes freely the air which 

 floats above. This remarable place is known under the name of 

 the Grnttn del Cunn, ttr dot/'s groftft.'] 



30. The air which escapes from the lungs is composed of the 

 nitrogen inspired, of a portion of oxygen not employed, and of 

 carbonic acid furnished by the act of respiration. 



31. The expired air is also loaded with vapor of water exhaled 

 from the blood during its passage through the capillary vessels of 

 the lungs. This vapor becomes very perceptible, when the cold 

 condenses it, at the moment of its issue from the body, and con 

 stitutes what physiologists call pulmonary trnuspiration. 



32. Since the air is quickly vitiated by respiration, and its oxy 

 gen disappears to be replaced by the carbonic acid, we readily 

 infer, that this fluid must be constantly renewed in the lungs, and 

 in fact that this takes place in consequence of the alternate 

 movements of inspiration and expiration. 



1 __ 



28. How is carbonic acid gas formed ? 



29. Upon what do certain accidents in mine*, caves, wells, &c. depend ? 

 What is the Grotto del Ctinn near Naples remarkable for? 



30 Of wh'.t is the air which escapes from the lungs composed? 



31. What is pulmonary transpiration? 



32. Why is it necessary te renew the air in the lungs ? 



