On the Physical Properties of Soil. 



181 



weight among tlie usual ingredients of soil ; in arable soils, 

 however, magnesia is not found in this fine form, but usually in 

 combination with lime or silica ; in these two combinations it has 

 a coarser form, the physical properties of which approach more 

 nearly to those of sand. 



6. Humus, among the usual constituents of soil, has the least 

 specific gravity, and, if we except the pure artificial magnesia, 

 it has likewise the least absolute weight. 



7. Compound arable soils are generally lighter in proportion as 

 they are richer in humus ; we must not, however, conclude posi- 

 tively from this intimation alone as to the fertility of a soil, since 

 the humus itself is liable to great differences, and even the other 

 pure earths exhibit, according to the fineness of their particles, 

 great diversity in weight, and consequently mixed earths may 

 acquire very different average weights ; a more certain evidence 

 on this point is furnished by the specific gravity than by the 

 absolute weight. 



8. The usual denomination given by the farmer of heavy or 

 light soils, refers neither to the specific gravity nor to the abso- 

 lute weight of the earth ; clay soils, in their dry as well as in 

 their wet state, are of less weight than sandy soils ; these designa- 

 tions, therefore, of heavy and light, refer much more to the dif- 

 ferent consistence of the earths, of which we shall say more 

 subsequently. 



Weight of artificial mixtures of earths. — When different earths 

 are artificially mixed together, a cubic inch of the earthy mixture 

 obtained gives a weight greater than the arithmetical mean (or 

 common average) of the earths entering into the mixture, whether 

 mixed in equal portions according to weight or volume, or in 

 other quantities. I took, in different proportions, a common 

 siliceous sand, a rich clay, and a fine clay-marl, of which I had pre- 

 viously ascertained the absolute weights, and mixed them together, 

 when I determined the weight of the mixture. I obtained the 

 following results : — 



Kinds of Earth. 



Weight of 5.7 



Arithmetical 



Increase of 



Cubic Inch. 



Mean. 



Weight. 





Grains. 



Grains. 



Grains, 





2840 









2020 









1790 







Clay and Sand in equal proportions by weight 



2545 



2430 



115 



Clay and Sand in equal proportions by volume 



2685 



2430 



255 



2 parts Clay and 1 part Sand by weight . . 



2390 . 



2293 



97 



2 parts Clay and I part Sand by volume . . 



2470 



2293 



177 



2 parts Sand and 1 part Clay by weight , . 



2740 



2566 



174 



2 parts Sand and 1 part Clay by volume . • 



2825 



2566 



259 



Equal parts of Marl and Sand by weight . . 



2315 



2267 



48 



