210 SOIL ITS ANALYSIS. 



indicated enables us to do so ; we have but to evaporate the liquit'. 

 from which the oxalate of lime was deposited, and then to calcine 

 the product of the evaporation in a platinum capsule. Any nitrate 

 of magnesia which may exist there will be decomposed at a dull red 

 heat, as well as any oxalate of ammonia which may have resulted 

 from ammonia added in excess. By treating the residue of the cal- 

 cination with water we obtain the magnesia, which being washed. 

 has only to be calcined and its weight ascertained by weighing. 



2d. If we would be ;ontent with a simple approximation, we may 

 judge of the quantity oi calcareous carbonate contained in a vegeta- 

 ble soil by measuring the quantity of carbonic acid which we obtain 

 from it. We counterpoise upon the scale of a balance a phial con- 

 taining some diluted nitric acid ; we weigh a certain quantity of the 

 earth to be analyzed, and this is added by degrees to the acid. If 

 the earth contains carbonates, effervescence ensues. The liquid is 

 shaken with care, and having waited a few minutes in order to let 

 the carbonic acid which is mixed with the air of the phial escape, 

 the phial with its contents is again put into the balance. If there 

 has been no disengagement of carbonic acid, it is clear that to restore 

 the equilibrium it will be sufficient to add to the opposite scale the 

 weight of the earth which was put into the phial ; whatever is want- 

 ing of this weight represents precisely the weight of carbonic acid 

 which has been disengaged. Presuming this acid to have been com- 

 bined with lime, the weight of the calcareous carbonate can be cal- 

 culated exactly. 



Sulphate of lime is an occasional constituent of soils ; to ascertain 

 its presence and quantity, the following is the method of procedure : 



The earth well pulverized is first roasted for a considerable time 

 in a crucible or platinum capsule, until all the organic matter is com- 

 pletely destroyed ; it is advisable to operate on about 100 grammes, 

 or about 3.2 ounces troy of soil. After this operation the matter is 

 boiled in 4 or 5 times its weight of distilled water for some time ; 

 water being added to replace that which is dissipated by evaporation ; 

 we then filter, re-wash, and having added all the liquors, we evapor- 

 ate in a capsule until the volume of the liquid is reduced to a few 

 drachms. To the liquid thus concentrated we add its own bulk of 

 alcohol. If the solution contains sulphate of lime it will be deposit- 

 ed, and the deposite being received upon a filter and washed with 

 weak alcohol, its weight is taken after having been dried and calcined. 

 This salt is frequently seen deposited in the form of fine colorless 

 needles on the cooling of the sufficiently concentrated solution ; but 

 the addition of alcohol is always useful, because the sulphate of 

 lime, which is not very soluble in water, is altogether insoluble in 

 weak spirit, which on the contrary dissolves certain alkaline and 

 earthy salts whose presence would interfere with the accuracy of 

 the result. 



It may be matter of great moment to determine the existence and 

 the quantity of phosphates contained in a soil destined for cultiva- 

 tion. Although the search for phosphoric acid may perhaps require 

 a certain familiarity with chemical analysis, I shall neverthelest 



