SOIL. 



The number of these grains at once 

 indicates the specific gravity of the 

 substance ; for the space above the 

 Wilier was that of 100 grains of water, 

 and this space is now tilled up by the 

 earth examined. Its weight, there- 

 fore, denotes its specific gravity com- 

 pared to water as 100 ; and as a del- 

 icate balance readily turns with a 

 decimal of a grain, the decimals give 

 the specific gravity to the third fig- 

 ure. Thus, if the grains are 256 

 and the decimals 4, the specific grav- 

 ity is accurately 2564, water being 

 1000. 



" By taking the specific gravity of 

 the pure earths, it is found that sili- 

 ca is the heaviest, the next is car- 

 bonate of lime, and the lightest is 

 alumma, while organic matter IS much 

 lighter than any earth. Thus, a tol- 

 erable guess can be made of the com- 

 position of that impalpable portion of 

 the soil which generally contains all 

 the salts and organic matter. To 

 separate these chemically requires 

 more experience and a more exten- 

 sive apparatus ; but the quantity of 

 carbonate of lime in any soil which 

 effervesces with acids, when its pres- 

 ence is thus ascertained, can be cal- 

 culated by a simple process, almost 

 as easily as the specific gravity ; for 

 this purpose, it is necessary to have 

 a balance with a somewhat larger 

 scale, in which can be conveniently 

 placed a small glass cup and a vial. 

 The cup is placed in the scale with 

 100 grains weight in it ; and the vial, 

 also containing 200 or 300 grains, 

 more or less, of very dilute muriatic 

 acid. The whole is accurately poised 

 Fine dry sand is the most convenient 

 counterpoise when the exact weight 

 is not required. The 100 grains are 

 now taken out and replaced by the 

 dried soil to be examined. When 

 the equilibrium is restored, the dilu- 

 ted muriatic acid is poured carefully 

 and gently on the earth in the cup 

 as long as it continues to effervesce ; 

 it is then left for a while, and a little 

 more of the acid added. If no bub- 

 bles appear, then all the carbonic acid 

 has been expelled, and the opposite 

 scale preponderates. The grams and 



Q Q Q 



decimals of grains required to restore 

 the equilibrium give the weight of the 

 carbonic acid expelled, which will 

 sometimes be considerable. Since 

 100 grains of carbonate of lime con- 

 tain 44 grains of carbonic acid, we 

 have only to take the proportion as 

 follows : let a denote the grains of 

 carbonic acid indicated in the experi- 



100a , 

 ment ; then, 44 : 100 : : a : -^^^the 



44 



quantity of carbonate of lime in the 

 soil ; that is, multiply the grains add- 

 ed to the scale by 100, and divide by 

 44. This experiment, repeated with 

 a very accurate balance, will surprise 

 by its correctness ; and no chemical 

 analysis could give it with equal cer- 

 tainty, especially in the hands of an 

 inexperienced person. When the 

 weights are ascertained by substitu- 

 tion, the accuracy of the balance is of 

 less consequence ; all that is required 

 is that it be sensitive, or turn read- 

 ily by the addition of very minute 

 weights. Thus, by two simple and 

 easy experiments, some of the most 

 important qualities of the impalpable 

 portions of a soil may be accurately 

 ascertained, viz., its specific gravity, 

 and the quantity of carbonate of lime 

 which it contains. 



" It is more difficult to separate the 

 fine silica from the alumina, and this 

 is of less importance than might be 

 supposed ; for silica, when extremely 

 divided, so as to remain long sus- 

 pended in water, and mixed with alu- 

 mina, becomes as impervious to wa- 

 ter as alumina itself, and therefore its 

 mechanical effect on the soil is the 

 same. For farther information, see 

 art. Analysis. 



" The coarser portions of the soil, 

 which have been separated in the 

 sifting, are easily examined by the 

 eye. If repeated washing carries off 

 nothing from them, they may be con- 

 sidered as so many crystals, which 

 have no other effect in the soil than 

 to keep it open. If some of these 

 are of a calcareous nature, they will 

 dissolve with effervescence in mu- 

 riatic acid, and their proportion can 

 be ascertained Ijy the process abovt 

 described ; if not, they may be a 



733 



