30 CARBONIC ACID PRESENT IN THE ATMOSPHERE. 



experiments have shown that it is always there in very 

 nearly the same proportion. This proportion is quite 

 small, being only jjg'gTjth of the whole bulk, or about 

 j^i,j jth of the whole weight. It seems insignificant, 

 too much so to be noticed; but when we come to cal- 

 culate from the known weight of the atmosphere on 

 each foot of the earth's surface, we find that there is in 

 the air over each acre of ground about seven tons of 

 this gas. This is a considerable quantity, and, when 

 calculated over the whole surface of the earth, amounts 

 to billions of tons. It is foimd to be just graduated 

 to the wants of both plants and animals. All living 

 things, as has been said, die in an atmosphere which 

 contains a large proportion of this gas. Plants, 

 however, require a certain portion of it to be spread 

 through the air, that they may draw it in through their 

 leaves. This is necessary to their life, as they will 

 not live for any length of time in an atmosphere where 

 there is no carbonic acid gas, and will not flourish if 

 the proportion of j^'o^jth be greatly reduced. On the 

 other hand, if this proportion be much increased, if 

 more carbonic acid be introduced into the air, the effect 

 is also injurious. The proportion of carbonic acid 

 may with benefit be increased, according to some 

 experiments, so long as the sun shines and daylight 

 continues. When the sun goes down, however, and 

 darkness comes over the earth, more of this gas than 

 is usually present does harm. We see then that the 

 Creator has regulated the quantity of carbonic acid, so 

 that there is just enough for the necessities of the plant, 

 and not so much as to injure either plants or animals, 

 while at the same time regard has been had to the 

 alternations of day and night. 



