CARBONIC ACID OF THE ATMOSPHERE. 85 



changed ; the latter increasing at the expense of the former. 

 But when we examine the atmosphere, we find that there is a 

 fixed relation between these two substances; one hundred 

 parts of air contain twenty parts of oxygen by volume in one 

 hundred, and from five to six y^^^ part of carbonic acid by vol- 

 ume or about yw<j^ P^^'t by weight. So that the air, at the 

 present day, is just as pure, as that which existed 4000 years 

 ago, and, although billions of cubic feet of carbonic acid are 

 thrown off into the air, and an equal volume of oxygen con- 

 sumed, (one man consuming 45,000 cubic inches per day,) 

 still, this relation is not disturbed. 



How is this acid disposed of? and from what source is the 

 oxygen derived to fill its place ? for a cubic foot of oxygen 

 gas, by uniting with carbon, so as to form carbonic acid, does 

 not change its volume. The billions of cubic feet of oxygen 

 extracted from the atmosphere, are replaced by the same num- 

 ber of billions of cubic feet of carbonic acid which immediate- 

 ly supply its place. There must be some cause, or causes, 

 which exists, capable, both of removing the carbonic acid, 

 and of replacing an equal volume of oxygen, which is removed 

 from the air, by the processes above described. This cause 

 is to be found, principally, in the process of vegetation. 



1. The carbonic acid of the atmosphere is absorbed and de- 

 composed by vegetables ; its carbon assimilated, and its oxy- 

 gen given back to the atmosphere. 



All the green parts of plants are capable of absorbing this 

 gas, but the property is mostly confined to the leaves, which 

 possess it, quite independent of the plant itself, as they per- 

 form the function of absorbing carbonic acid, and emitting 

 oxygen when separated from the stalk. The upper surfaces 

 of leaves appear to have peculiar organs of absorption, as they 

 will not perform this function when bruised. The presence 

 of light is necessary, in order that the leaves may decompose 

 carbonic acid. This fact was first proved by Ingenhouse, 

 who also found, that plants emit no oxygen gas, when made 



