THE SOIL PARTICLE 69 



ing from fine gravel, readily visible to the naked eye, to the 

 clay separate, the largest particle of which is .005 of a milli- 

 meter or .0002 of an inch in diameter. The stone and large 

 gravel, while they figure in a practical examination and de- 

 scription of a soil in the field, obviously need not be considered 

 in such a classification as this. 



The seven separates may be thrown into two groups for 

 a preliminary examination on the basis of visibility to the 

 naked eye. The gravel and sand particles are readily seen, 

 while the silt and especially the clay particles are invisible 

 as individuals, although some of the larger silt particles may 

 be seen with the naked eye when suspended in water. The 

 gravel and sand, when dominant in a soil, give properties 

 known to every one as sandy, while if the soil is made up 

 largely of silt and clay, its plasticity and stickiness proclaim it 

 as clayey in nature. The characteristics of the two soils of the 

 above table may be read easily from their mechanical analyses. 

 The classification, therefore, meets the criteria already estab- 

 lished. It is simple, easy to remember, and is capable of 

 expressing, to a certain extent at least, the dominant physical 

 characters of soils. As will be shown below, it lends itself 

 to the quantitative separation of a soil, the so-called mechan- 

 ical analysis. 



39. The beaker metliod of mechanical analysis. — ^When 

 fragments of rock or soil are suspended in water they tend 

 to sink slowly, and it is a well recognized fact that, other 

 things being equal, the rate of settling depends on the size 

 of the particle. As the particle is decreased in size its weight 

 decreases faster than the surface exposed to the buoyant force 

 of the water. As a consequence the rapidity with which the 

 soil particles settle is more or less proportional to their size. 

 The suspension of a sample of soil would, therefore, be the 

 first step in mechanical separation by water ; the second step 

 would be subsidence and the withdrawal of each successive 

 grade of particles as it slowly settled; the third step would 



