114 PHYSIOLOGY OF 



The leaves, in the sunshine, uniformly absorb car- 

 bonic acid gas from the atmosphere, by their under sur- 

 faces, and decomposing it, retain the carbon and give 

 out its oxygen. The carbon retained, no doubt goes 

 into the juices in the leaves. This, it will be perceiv- 

 ed, is the converse of what is effected on the atmos- 

 phere by the lungs of animals. Animals absorb oxy- 

 gen and produce carbonic acid gas, but vegetables ab- 

 sorb carbonic acid, and give out oxygen gas ; thus ani- 

 mals and vegetables serve, very happily, to counteract 

 each other's effects on the atmosphere. 



In animals, the blood in the lungs, by parting with 

 some of its carbon, changes from a purple to a vermil- 

 lion colour. In the leaves, the juices from being col- 

 ourless, become green, not by losing carbon, for that 

 principle is increased in them. 



So great is the quantity of carbonic acid gas pro- 

 duced by the breathing of animals, and various chemic- 

 al operations in nature, that we should probably be 

 suffocated by it were it not continually decomposed by 

 the whole world of vegetables, and the pure vital air 

 restored for our respiration. 



Leaves in the night give out carbonic acid, but in a 

 less quantity than that which they absorb during the 

 day. Flowers are found to give out carbonic acid 

 both in the sunshine and shade, of course their effect 

 is uniformly to vitiate the air. 



Observation. 1. It will be perceived, from what takes place in 

 leaves, that plants derive a part of their solid nourishment from the 

 air, that is, the carbon or charcoal matter from the decomposed 

 fixed air or carbonic acid gas. 



2. Some inquiries of Mr. Ellis of Edinburgh go to prove, that 

 vegetating plants, at all times, absorb oxygen and produce carbo- 

 nic acid in its stead Smith** Bat, p. 175. Note. 



The leaves are also organs of perspiration. The 

 blood or juices of plants give off a portion of their water 

 in the form of perspiration, through the under surface of 

 the leaves. Dr. Hales proved that an annual Sunflower 

 perspired in dry weather, more than a man. The Cor- 



