152 HOW CROPS PEED. 



having all tlie same specific gravity, it would be possible 

 by tlie use of a properly constructed washing apparatus 

 to separate a sample into fifty or one hundred parts, and 

 to define the dimensions of the particles of each of these 

 parts. Since, however, the soil is very heterogeneous, and 

 since its particles are unlike in shape, consisting partly of 

 nearly spherical grains and partly of plates or scales upon 

 which moving water exerts an unequal floating effect, it is 

 difficult, if not impossible, to realize so perfect a mechanic- 

 al analysis. It is, however, easy to make a separation of a 

 soil into a large number of jjarts, each of which shall ad- 

 mit of precise definition in terms of the rapidity of flow 

 of a current of water capable of sustaining the particles 

 which compose it. Instruments for mechanical analysis, 

 which provide for producing and maintaining at will any- 

 desired rate of flow in a stream of water, have been very 

 recently devised, independently of each other, by E. Schone 

 {loc. cit., pp. 331-405) and A. Muller ( Vs. St., X, 25-51). 

 The employment of such apparatus promises valuable re- 

 sults, although as yet no extended investigations made 

 with its help have been published. 



Gravelly Soils are so named from the abundance of 

 small stones or pebbles in them. This name alone gives 

 but little idea of the really important characters of the 

 soil. Simple gravel is nearly valueless for agricultural 

 purposes ; many highly gravelly soils are, however, very ' 

 fertile. The fine portion of the soil gives them their crop- 

 feeding power. The coarse parts ensure drainage and 

 store the solar heat. The mineralogical chaiacters of the 

 pebbles in a soil, as determined by a j)ractised eye, may 

 often give useful indications of its composition, since it 

 is generally true that the finer parts of the soil agree in 

 this respect with the coarser, or, if d fferent, are not in- 

 ferior. Thus if the gravel of a soil contains many pebbles 

 of feldspar, the soil itself may be concluded to be well 

 supplied with alkalies; if the gravel consists of limestone, 



