PRESSURE OF THE ATMOSPHERE. 89 



plants vegetate much better in the sun than when placed in 

 common air, but a larger quantity proves injurious, and when 

 an atmosphere contains three fourths of its volume of that gas, 

 plants will not vegetate at all, any more than they will in the 

 pure acid. Any addition of the gas in the shade, retards ra- 

 ther than promotes vegetation. Sennebier has shown, that 

 the green color of plants depends upon the absorption of car- 

 bonic acid ; for this purpose light and oxygen gas must also 

 be present. 



It is a fact worthy of notice, in this connection, that carbon^ 

 ic acid, which is so essential to vegetation, and so grateful to 

 be used in solution, as a drink, acts, nevertheless,, as a slow 

 poison when taken, much diluted with air, into the lungs of an- 

 imals, and produces almost instant death, when introduced 

 in a pure state. 



As it is produced by ordinary combustion, every year adds 

 many melancholy examples of its fatal power, in the case of 

 those who are so imprudent, as to use live coals to warm 

 sleeping apartments, which are not properly ventilated, or who 

 use furnaces for ironing, and for culinary purposes. The acid, 

 being heavier than the air, fills the apartment like water, and 

 as soon as the individual dips his head into it, he is suffocated 

 almost as soon as if plunged into water. 

 VII. 3Iechanical agency of the atmosphere. 

 1. The pressure of the air is of the highest importance to 

 the vegetable kingdom. As the pressure is about lolbs. upon 

 every square inch of surface, its force upon the leaves and 

 other parts of plants must be very great. It brings the oxy- 

 gen, carbonic acid and ammonia into direct contact with the 

 various organs of absorption. It also furnishes support to the 

 external surface of plants, and enables them to withstand the 

 pressure of the fluids within ; for were the atmosphere very 

 rare, or wholly removed, the processes within the plant would 

 injure or burst the vessels, and decay and death would ensue. 

 As we ascend above the level of the ocean this pressure 

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