NATURAL THEOLOGY. 



air is more than ordinarily nutritious to the plant/ 

 Agitation with water turns out to be another of 

 these restoratives. The foulest air, shaken in a 

 bottle with water for a sufficient length of time, 



^ The experiments of Priestley, confirmed by those of Ingen- 

 houz and Saussure, led to the conclusion that the air is kept pnre 

 by the action of plants, these emitting more oxygen gas by day 

 than carbonic acid gas by night. Some doubt was thrown upon 

 this point by the experiments of Mr. Ellis ; but these appear to 

 have been removed by Sir H. Davy's remarks and experiments. 

 It seems probable that the plant, growing in the light, decomposes 

 both the carbonic acid which exists, though in a very small pro- 

 portion, in the atmosphere, and also any that may exist in the wa- 

 ter applied to its leaves ; for Sennebier found that when these are 

 immersed in water impregnated with carbonic acid, oxygen gas 

 was evolved, but not if water was used which had been boiled. 



The process of vegetation appears to be the great means of sup- 

 plying the loss of oxygen in the atmosphere ; indeed, none other 

 have been as yet discovered. The composition of the atmosphere, 

 in respect of purity, though at one time supposed to vary in dif- 

 ferent places, is now ascertained to be every where invariably the 

 same. It contains about 20 parts of oxygen gas, by measure, in 

 100, and the remaining 80 are almost entirely nitrogen gas. The 

 only variation is in the slight portion of carbonic acid gas, which 

 never exceeds 1 in 100, and is seldom more .than I in 1000, sup- 

 posing the circulation of the air to be unconfined. But the pro- 

 portion of oxygen to the whole bulk has been found to be the same 

 at nearly 22,000 feet high, and in the deepest valleys ; the same 

 in countries widely remote from each other, and differing in cli- 

 mate as well as soil ; the same in the most pestilential marshes, or 

 in hospitals, and in the most open and healthy situations. The 

 agitation of the air by winds speedily mixes all its strata, and pre- 

 vents the effects of breathing and burning from being perceived. 

 Yet a constant consumption of oxygen is going on, wherever 

 there are either living creatures of the hot-blooded class, or fires of 

 any kind, natural or artificial ; and this is so balanced by the pro- 



2* 



