32 LESSONS IN CHEMISTRY. 



driven off, the rock loses nearly half its weight, and the es- 

 caping gas has frequently proved fatal to persons who have 

 fallen asleep near the place where lime was being burned. 



26. Carbonic acid gas is also given out in enormous quan- 

 tities from the lungs of animals in breathing. The air which, 

 in a single expiration, we expel from our lungs contains from 

 3j to 4 per cent of it, and it has been calculated that the 

 air which, in the course of a day, is expired by a full-grown 

 man, actively employed, will yield as much of this gas as 

 would be produced by burning 1 3 oz. of charcoal in an open 

 fire. It is also, like ammonia, evolved wherever animal or 

 vegetable substances are undergoing decomposition (15); so 

 that when farm-yard manure, or vegetable matters of any 

 kind, are mixed with the soil of your cultivated fields, a 

 gradual and continued supply of both ammonia and carbonic 

 aeid is produced. But though carbonic acid is thus from so 

 many sources continually escaping into the atmosphere, we 

 find as has been stated (24), that it constitutes but a small 

 part of its bulk. I will have occasion, in a subsequent 

 chapter, to explain how the accumulation of this gas, so in- 

 jurious to animal life, is prevented, and its production made 

 to contribute to the support of the crops which you cultivate. 



27. The examination of the materials of which our culti- 

 vated plants are composed shows us that, when deprived of 

 water, nearly 50 per cent, of their weight consists of carbon. 

 But, as it has already been stated (21) that that substance 

 is insoluble in water, it is evident that it cannot, in its ordi- 

 nary form, be taken up by vegetables. It is necessary that 

 it should in some way be rendered soluble. This Nature 

 effects by combining it with oxygen to produce the gas 

 which I have just been describing ; for carbonic acid dissolves 

 readily in water, the agreeable taste of spring water and 

 several fermented liquors being due to its presence. It is not 

 only soluble in water, but possesses, when in solution, the 

 power of dissolving lime and several other bodies not capable 

 of solution in pure water. ^ We know that it is by the sol- 

 vent action of this gas contained in the streams which trickle 

 over rocks containing lime that that earth is dissolved, and 

 communicates hardness to our springs.* When boiled, the 



* If a current of carbonic acid be allowed to pass for some time 

 through the milky liquid described in note c, page 31, it will gradually- 

 become clear, carbonate of lime being soluble in an excess of this gas. 



