PURE AIR 101 



the tissues of our bodies does not produce flame as 

 in the stove or furnace, for the combustion is slow 

 and more like that which causes rusting of iron and 

 decay of wood. Without this useful oxygen the Won- 

 derful House that Jack Has would be supplied with 

 neither heat nor energy, and, as a consequence, its 

 destruction would soon follow. 



As far as is known, nitrogen passes out of the body 

 without any change, its only apparent use being to 

 dilute the oxygen. This, however, is a very important 

 service. As we shall see later, if oxygen were much 

 stronger than it is in normal air, its union with carbon 

 would take place too fast. 



But what about the carbonic acid gas, or carbon 

 dioxid as it is often called ? We have found out that 

 less than one-hundredth of the air is watery vapor and 

 this last named gas. Have chemists really discovered 

 how much of the latter there is in pure air? There is 

 such a small amount of it, compared with the whole 

 body of atmosphere, that it does seem almost magical 

 that an accurate estimate could be made. Never- 

 theless, chemists in various parts of the world agree 

 in the opinion that there is only about one part of 

 carbonic acid gas to every twenty-five hundred parts of 

 air. While this gas is a deadly poison, the very small 

 proportion in pure air has no ill effect when taken into 

 the lungs. The best authorities, however, are agreed 

 that twelve parts of carbonic acid gas to ten thousand 

 of air is the upper limit of safety, and that twenty-two 



