386 DETAILED SYNTHESIS. 



supposing that muscular exercise is useful chiefly in reference to the 

 muscles rather than by increasing respiration ; while in fact the rela- 

 tions of air to the Blood are more constant,^piore extensive, and more 

 effective upon health, than those of any other thing, even food. 



The Atmosphere. 



1246. THE ATMOSPHERE, WHEN PURE, is COMPOSED 

 of nearly four-fifths nitrogen, one-fifth oxygen, a trace 

 of carbonic acid, and a variable quantity of watery va- 

 por, that should be near the point of saturation. 



1247. THE TEMPERATURE OF THE ATMOSPHERE af- 

 fects its action in the lungs in two ways. The colder 

 the air, the more of it in a given space, and the more it 

 expands in the lungs, filling its cells and acting more 

 completely on the Blood circulating about them ; the 

 warmer the air, the less of it in a given space, and the 

 less it expands in the lungs. 



1248. Inf. In winter, when more heat should be produced in the 

 body, the air inhaled is adapted to that purpose. 



1249. AIR THAT HAS BEEN BREATHED once, contains 

 about one-fifth less oxygen than pure air ; carbonic acid, 

 equal to the loss of oxygen ; a variable quantity of other 

 impurities ; and is saturated with moisture. 



1250. THE AMOUNT OF OXYGEN LOST, AND CARBONIC 

 ACID GAINED, by the breath, depends upon the purity 

 and temperature of the air, the condition of the Blood, 

 the activity of its circulation, the vigor of respiration, 

 and the health of the body. 



1251. Inf. THE AIR IN WHICH A PERSON RESPIRES is, with every 

 breath, becoming unhealthy, from diminished oxygen, and from in- 

 creased carbonic acid, various other impurities, and moisture. 



1252. Remark. From the skin many of the most noxious exhala- 

 tions are constantly thrown into the air. Indeed, from this source 

 there is so much that is poisonous, that when large numbers of persons 

 live compactly, disease is very apt to be developed, as on shipboard, in 

 tenant-houses, and even in schools and colleges. 



1246. How is ? 1247. What said of? 1248. Relation of air to winter. 

 1249. What said of - ? 1250. What said of ? 1251. What said of - ? 1252. Remark. 



