RESPIRATION 193 



on this subject by Prof. Tyndall, he remarks that, " by breath- 

 ing through a cotton wool respirator, the noxious air of the 

 sick-room is restored to practical purity. Thus filtered, attend- 

 ants may breathe the air unharmed. 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 off by filters of cotton wool. By this means, so far as 

 the germs are concerned, the air of the highest Alps may be 

 brought into the chamber of the invalid." 



32. Carbonic Acid Gas 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 atmosphere in the pro- 

 portion 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 hollows 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 animals, which were suffocated by carbonic acid 

 gas while passing over it. The Lake Avernus, the fabled 

 entrance to the infernal regions, 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 

 gas forms the dreaded choke-damp, while carburetted hydrogen 

 is the fire-damp. 



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

 gas, for, as we shall see presently, nature provides for its rapid 

 distribution, and even turns it to a good use. But its ill effects 

 are painfully evident in our homes, schools, and churches, 

 where 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 



than one-half cubic foot of carbonic acid gas per 



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



33. In the open air ? Amount of carbonic acid exhaled by 

 room fire ? From furnaces ? 



