142 PHYSIOLOGY AND HEALTH. 



316. We experience faintness and languor in the warm 

 season, because the air does not purify a sufficiency of blood 

 for the vigorous sustenance of the system. Then people 

 complain that " the air is heavy," which is directly opposite 

 to the truth, for the air is really light, and it does not contain 

 sufficient oxygen to invigorate them, and give sufficient 

 strength and elasticity to bear the burdens and operations of 

 life easily, and hence all these are heavy to them. We ob- 

 serve animals puff and breathe rapidly after running in sum- 

 mer, and we do the same on any active exertion, more in 

 warm than in cold weather. We do this to bring a more 

 frequent supply of air to the blood, and thus to compensate 

 for the lightness of the air, and deficiency of oxygen, by the 

 rapid renewal of both. 



317. The air is more dense in the lower regions of the 

 earth, and on the level of the sea, than on the heights of 

 mountains. As we ascend from below to the higher eleva- 

 tions, we find the air lighter and more expanded, and we are 

 compelled to breathe more rapidly. Travellers all complain 

 of the increasing languor and faintness as they ascend, and 

 enjoy the bracing and invigorating effect of the air as they 

 come down the mountains. 



318. All kinds of impurities in the air, and every thing 

 that diminishes the proportion of oxygen, have the same 

 effect of weakening or diminishing the vital properties of the 

 air. In some mines, a gas is given out, called ihejire damp, 

 which is carburetted hydrogen. Whenever this is present, 

 it lessens the vivifying power of the air, by excluding its oxy- 

 gen. If it is breathed in small quantities, it occasions gid- 

 diness, sickness, and diminished nervous power ; and, when 

 it is in great proportion, the miners are unable to breathe, 

 and often fall a sacrifice to it. 



319. The general state of the system affects the quantity 

 of matter which is carried out through the lungs. When 

 the whole frame is well, and all the functions are carried on 

 vigorously, when the circulation of the blood is easy and 

 the respiration well sustained, then the old particles of the 



