LECTURE 111. 



Carbonic and oxalic acids, their properties and relations to vegetable life—Carbonic oxide 

 and iigiit carburetted hydrogen, their properties and production in nature— Ammonia, its 

 properties and relations to vegetable life. 



§ 1. Carbonic acid, its properties and relations to vegetable life. 



When charcoal is burned in the air it combines slowly with oxygen, 

 and is transformed into carbonic acid gas. In oxygen gas it burns more 

 rapidly and vividly, producing the same compound. 



This gas is colourless, like oxygen, hydrogen, and nitrogen, but is 

 readily distinguished from all these, by its acid taste and smell, by its solu- 

 bility in water, by its great density, and by its reddening vegetable blues. 

 Water at 60 F. and under the ordinary pressure of the atmosphere, dis- 

 solves rather more than its own bulk of this gas (100 dissolve 106), and, 

 however the pressure may be increased, it still dissolves the same bulk. 



All gases diminish in bulk uniformly as the pressure to which they 

 are subjected is increased. Thus under a pressure of two atmospheres 

 they are reduced to one-half their bulk, of three atmospheres to one- 

 third, and so on. When water, therefore, is saturated with carbonic 

 acid under great pressure, as in the manufacture of soda water, though 

 it still dissolves only its own bulk, yet it retains a weight of the gas 

 which is proportioned to the pressure applied. For the same reason 

 also, when the pressure is removed, as in drawing the cork from a bot- 

 tle of water so impregnated, the gas expands and escapes, causing a 

 lively effervescence, and the water retains only its own bulk at the ex- 

 isting pressure. This solution in water has a slightly sour taste, and 

 reddens vegetable blues. These properties it owes to the presence of 

 the gas, which is therefore what chemists call an acid body, and hence its 

 name of carbonic acid. [Acids have generally a sour taste, redden 

 vegetable blues, or combine with bases, such as lime, soda, potash, &c., 

 to form salts.] 



This gas is one-half heavier than atmospheric air, its density being 

 1'524, and hence it may be poured through the air from one vessel to 

 another. Hence also, when it is evolved from crevices in the earth, in 

 caves, in wells, or in the soil, this gas diffuses itself through the atmos- 

 phere and ascends into the air, much more slowly than ihe elementary 

 gases described in the previous lecture. Where it issues from the earth 

 in large quantity, as in many volcanic districts, it flows along the surface 

 like water, enters into and fills up cracks and hollows, and sometimes 

 reaches to a considerable distance from its source, before it is lost among 

 the still air. 



Burning bodies are extinguished in carbonic acid, and living beings, 

 plunged into it, instantly cease to breathe. Mixed with one-ninth of its 

 bulk of this gas the atmospheric air is rendered unfit for respiration. It 

 is, however, the principal food of plants, being absorbed by their leaves 

 and roots in large quantity. Hence the presence of carbonic acid in the 

 atmosphere is necessary to the growth of plants, and they have beenob- 

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