RESPIRATION. 349 



much at different times and in different places, being heav- 

 ier, on a clear than on a close day, and also in low places 

 than in lofty ones. ' The small portion of carbonic acid gas , 

 which the atmosphere contains is not chemically, but me- 

 chanically mixed with it. This gas is evolved by the 

 fermentation of beer, and the decomposition of vegetables, 

 and is often found in wells and deep places. It is much 

 heavier than the atmosphere, and thus remains in these low 

 places by its gravity. A lighted candle placed in this gas 

 is immediately extinguished ; so that it is used as a safe- 

 guard in descending into these low and foul places ; for 

 whatever will not support combustion will not support life. 

 It is not a simple gas, like oxygen, but is formed by the 

 union of carbon and oxygen. 



Nitrogen or azote is a simple gas, but its use in the at- 

 mosphere seems J;o be principally of a passive nature, being, 

 for the purpose of diluting the oxygen and rendering it less 

 stimulating : it will not alone support life or combustion, but 

 is chemically mixed with the oxygen. Oxygen is essential 

 for the support of life and combustion ; for if air be deprived 

 of it no animal can live, nor will a candle remain lighted 

 It is abundantly furnished by plants and shrubs, which thus 

 restore the loss of it occasioned by animals. When a flame 

 is exposed to this gas it greatly increases in brilliancy ; and 

 when venous blood is submitted to it, it quickly becomes florid. 



We have before shown that all the blood in the body was 

 in its turn carried from the heart to the lungs by means of 

 the pulmonary artery, which divides ^nd subdivides into the 

 smallest branches, and terminates in small capillary veins, 

 which, coalescing, become larger, and convey the blood 

 again to the heart by the pulmonary veins. Before it reaches 

 these veins, however, an important change takes place : the 

 blood proceeds from the heart in a black and impure state ; 

 . it returns reddened and purified ; it is submitted in its course 

 to the action of the air in the air-cells, not by actual contact, 

 but through the membrane which forms these cells : and by 

 this means the important change is effected. 



There is, we well know," a considerable difference be- 

 tween the expired and the inspired air ; the former is hot, 

 the latter cold ; this is healthy, that injurious ; one will sup- 

 port combustion and life, the other is unfit for breathing, and 

 will extinguish a flame. There is but little difference in 

 quantity between the air in its different states, but the oxjr 



