178 CONDITIONS REQUIRED FOR 



the lungs of a dark colour and unfit for the support 

 of life, assumes a florid red hue, and acquires the 

 power of supporting life. 



It is not easy to offer a satisfactory explanation 

 of the processes by which these changes are ef- 

 fected in the lungs. According to one view, the 

 carbonic acid contained in expired air is formed by 

 the secretion of carbon from the venous blood in its 

 passage through the lungs, which immediately unites 

 with the oxygen of the air, and forms carbonic acid, 

 in which shape it is then thrown out in expiration. 

 According to the other view, the carbonic acid exists 

 in, and is separated from, the venous blood in the 

 state of acid, and the oxygen which disappears is 

 absorbed into the circulating current. The former 

 explanation was long received, but Dr. Edwards has 

 lately advanced very strong grounds for adopting 

 the latter. Whatever may be the true theory, all 

 physiologists are agreed as to the fact that the arte- 

 rialization of the blood in the lungs is essentially 

 dependent on the supply of oxygen contained in the 

 air which we breathe, and that air is fit or unfit for 

 respiration in exact proportion as its quantity of 

 oxygen approaches to, or differs from, that con- 

 tained in pure air. If, consequently, we attempt to 

 breathe nitrogen, hydrogen, or any other gas not 

 containing oxygen, the result will be speedy suffo- 

 cation ; whereas, if we breathe air containing a too 

 high proportion of oxygen, the vital powers will 

 speedily suffer from excess of stimulus. From oxy- 

 gen being thus essential to life and respiration, it is 

 often called vital air, in contradistinction to those 

 gases which are incapable of supporting life. 



We can now appreciate the importance of a due 

 supply of fresh air wherever living beings are con- 

 gregated. In man, the rate of vitiation produced by 

 breathing, and the relative importance of ventila- 

 tion, may be easily estimated. An individual is 

 Ascertained to breathe, on an average, from 14 to 20 



