CALCAREOUS EARTH. 21 



most abundant agricultural earth. It is a combination of the che- 

 mical earth lime with carbonic acid, in the constant proportions 

 (in whole numbers) of 56 parts lime to 44 of carbonic acid. It is 

 converted to pure or quick-lime by red heat, which drives off the 

 carbonic acid ; and quick-lime, by exposure, and attracting carbonic 

 acid from the atmosphere, soon reverts to its original condition of 

 carbonate, or calcareous earth. It forms marble, limestone, chalk, 

 and shells, with very small admixtures of other materials. Thus 

 the term calcareous earth will not be used here to include either 

 lime in its pure state, or any of the numerous combinations which 

 lime forms with the various acids, except the one combination 

 (carbonate of lime) which is beyond comparison the most abundant 

 throughout the world, and most important as an ingredient of soils. 

 Pure lime attracts all acids so powerfully, that it is never presented 

 by nature except in combination with some one of them, and generally 

 with the carbonic acid. When this compound is thrown into any 

 stronger acid, as the muriatic, nitric, or even common vinegar, the 

 lime, being more powerfully attracted, unites with and is dissolved 

 by the stronger acid, and lets go the carbonic, which escapes with 

 effervescence in the form of air. In this manner, the carbonate of 

 lime, or ^calcareous earth, may not only be easily distinguished 

 from silicious and aluminous earth, but also from all other com- 

 binations of lime. 



The foregoing definition of calcareous earth, which confines that 

 term to the carbonate of lime, is certainly liable to objections, but 

 less so than any other designation. It may at first seem improper 

 and even absurd to consider as one of the principal earths which 

 compose soils, one only of the many combinations of lime, rather 

 than either pure lime alone, or lime in all its combinations. One 

 or the other of these significations is adopted by the highest autho- 

 rities, when the calcareous, ingredients of soils are described ; and 

 in either sense, the use of this term is more conformable with 

 scientific arrangement than mine. Yet much inconvenience is 

 caused by thus applying the term calcareous earth. If applied to 



understood and remembered as it is useful. To avoid repeated explana- 

 tions in the course of this essay, the rule will now be stated by which these 

 compounds are named. The termination of the name of the acid is changed 

 to the syllable ate, and then prefixed t the particular earth, alkali, or 

 metal with which the acid is united. With this explanation, any reader 

 can at once understand what is meant by each of some thousands of terms, 

 none of which might have been heard of before, and which (without this 

 manner of being named) would be too numerous to be fixed in the most 

 retentive memory. Thus, it will be readily understood that the carbonate 

 of magnesia i? a compound of the carbonic acid and magnesia the sulphate 

 of lime a compound of sulphuric acid and lime the sulphate of iron a com- 

 pound of sulphuric acid and iron and in like manner for all other terms 

 so formed. 



