152 HOW CROPS FEED. 
having all the same specific gravity, it would be possible 
by the 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 parts, 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. Schéne 
(loc. cit., pp. 334-405) and A. Muller (Vs. S¢., 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 characters of the 
pebbles in a soil, as determined by a practised 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 different, 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, 
