TESTING THE PRESENCE OP CALX. 57 



1 



certain invariable proportions of its two component parts; and 

 when all the lime has been precipitated as above directed, it will 

 necessarily be combined with precisely its first quantity of carbonic 

 acid. 



This operation is so simple, and the means for conductipg it so 

 easy to obtain, that it will generally be the most convenient mode 

 for finding the proportion of calcareous earth in those manures 

 that are known to contain it abundantly, and where an error of a 

 few grains cannot be very material; But if a very accurate result 

 is necessary, this method will not serve, on account of several 

 causes of error which always occur. Should no calcareous earth be 

 present in a soil thus analyzed, the muriatic acid will take up a 

 small quantity of aluminous earth, which will be precipitated by 

 the carbonate of potash, and without further investigation, would 

 be considered as so much calcareous earth. And if any compounds 

 of lime and vegetable acids are present (which, for reasons hereafter 

 to be stated, I believe to be not uncommon in soils), some portion 

 of these may be dissolved, and appear in the result as carbonate of 

 lime, though not an atom of that substance was in the soil. Thus, 

 every soil examined by this method of solution and .precipitation 

 will yield some small result of what would appear as carbonate, of 

 lime, though actually destitute of that ingredient. -The inaccura- 

 cies of this method were no doubt known (though passed over 

 without notice) by Davy, and other men of science who have 

 recommended its use ; but as they considered calcareous earth 

 merely as one of the earthy ingredients of soil, operating me- 

 chanically (as do sand and clay), on the texture of the soil, they 

 would scarcely suppose that a difference of a grain or two could 

 materially affect the practical value of an analysis, or the character 

 of the soil under examination.* 



The pneumatic apparatus proposed by Davy, as another means 

 for showing the proportion of calcareous earth in soils, is liable to 

 none of these objections; and when some other causes of error, 

 peculiar to this method, are known and guarded against, its accu- 

 racy is almost perfect, in ascertaining the quantity of calcareous 

 earth to which substance alone its use is limited. 



* "Chalks, calcareous marls, or powdered limestone, act merely by form- 

 ing a useful earthy ingredient in the sott, and their efficacy is proportioned 

 to the deficiency of calcareous matter, which in Larger or smaller quantities 

 seems to be an essential ingredient of all fertile soils ; necessary perhaps to 

 their proper texture, and as an ingredient in the organs of plants." [Da- 

 vy's Agr. Chem. page 21 and further on he says] "Chalk and marl or 

 carbonate of lime only improve the texture of a soil, or its relation to absorp- 

 tion,- it acts merely as one of its earthy ingredients" [It is evident, from these 

 expressions, that Davy considered calcareous earth important only as a 

 physical constituent of soils ; and it does not appear that he had any con- 

 ception of its far more important and useful service, in very minute pro- 

 portions, as a chemical agent, essential to fertilization.] 



