138 RESPIRATION. 



In all probability, the protection of the lungs will be the 

 protection of the whole system. For it is exceedingly 

 probable that the germs which lodge in the air-passages 

 are those which sow epidemic disease in the body. If this 

 be so, then disease can certainly be warded oif by niters of 

 cotton wool. By this means, so far as the germs are con- 

 cerned, the air of the highest Alps may be brought into 

 the chamber of the invalid." 



32. Carbonic Acid in the Air. We have already 

 spoken of this gas as -an exhalation from the lungs, and a 

 source of impurity; but it exists naturally in the atmos- 

 phere in the proportion of one half part per thousand. In 

 volcanic regions it is poured forth in enormous quantities 

 from fissures in the earth's surface. Being heavier than 

 air, it sometimes settles into caves and depressions in the 

 surface. It is stated that in the island of Java, there is a 

 place called the " Valley of Poison," where the ground is 

 covered with the bones of birds, tigers, and other wild ani- 

 mals, which were suffocated by carbonic acid while passing. 

 The Lake Avernus, the fabled entrance to the infernal re- 

 gions, was, as its name implies, bird-less, because the birds, 

 while flying over it, were poisoned by the gas and fell dead 

 into its waters. In mines, carbonic acid forms the dreaded 

 choke-dam}), while carburetted hydrogen is the fire-damp. 



33. In the open air, men seldom suffer from carbonic 

 acid, for, as we shall see presently, nature provides for its 

 rapid distribution, and even turns it to profitable use. 

 But its ill effects are painfully evident in the abodes of 

 men, in which it is liable to collect as the waste product of 

 respiration and of that combustion which is necessary for 

 lighting and warming our homes. A man exhales, during 

 repose, not less than one-half cubic foot of carbonic acid 

 per hour. One gas-burner liberates five cubic feet in the 



32. Carbonic acid in volcanic regions ? In Java? At Lake Avernus ? In mines? 



33. In the open air? Amount of carbonic acid exhaled by a man? A gas- 

 burner ? A room fire ? From furnaces ? 



