ACTION OF CARBONIC ACID. 63 



growth ; for it is a well established law of vegetable growth 

 that plants will not and cannot take into their circulation, 

 to any considerable extent, any larger proportion of any 

 one element of their structure than the normal quantity as 

 found existing in them upon chemical analysis. Thus 

 wheat plants contain certain elements in their composition,* 

 and these are found to be constant under all circumstances; 

 and notwithstanding that the soil might contain an excess- 

 ive quantity of any one of these elements, yet no more than 

 the normal proportion would be taken up by the wheat. 

 If one is increased, every one must be, and thus an increase 

 of one would necessitate an increase of all. If then the at- 

 mosphere should contain an excessive quantity of carbonic 

 acid and the growth of vegetation should be greatly stimu- 

 lated thereby, it would lead to a very rapid exhaustion of 

 the soil by the removal of the necessary mineral elements. 

 This principle is a fundamental one, and applies generally 

 to the growth of farm crops and should therefore be kept in 

 constant remembrance by every farmer. 



Carbonic acid unites with all the alkaline minerals in the 

 soil: as lime; magnesia; potash; soda; also w r ith ammonia; 

 as the carbonates of these substances. Its solution in water 

 gives this liquid an increased solvent power over mineral 

 substances; thus common carbonate of lime is practically 

 insoluble in pure water; but when the w r ater contains car- 

 bonic acid, it is able to dissolve a considerable quantity of 

 it, and this property applies to other mineral substances as 

 well. This gives a practical importance to the functions of 

 this acid which is of the greatest interest to cultivators of 

 the soil. A simple experiment will illustrate this behavior 

 of carbonic acid. A current of this gas passed through lime 

 water will produce a milky appearance in it by the forma- 

 tion and precipitation of carbonate of lime. After a short 

 time the cloudiness will disappear by the solution of the 

 carbonate thus formed, in the acid water. By heating the 

 water the carbonic acid is driven off and the carbonate of 

 lime is again precipitated and appears. 



The carbonic acid of the air is produced from a variety 



