90 MECHANICAL ANAL YSIS OF SOIL 



method.^ Ten grams are used for each experiment, 

 and the material, washed free from acid, is boiled with 

 water in a porcelain dish for one hour, with constant 

 stirring and working with the rubber pestle to break up 

 the small lumps. The soil, thus treated, is introduced 

 into the apparatus, the water allowed to enter slowly, 

 care being taken to avoid reaching even the lowest 

 required velocity until the outflow begins. 



The water is then regulated by means of the tap 

 D, so that the pressure in the piezometer is 2 cm., at 

 which pressure all that passes over may be regarded 

 as "clay." This may be collected and estimated by 

 evaporating an aliquot part to dryness ; but the clay 

 may be estimated by difference, in which case the 

 washings can be thrown away. 



When the runnings are perfectly clear, the different 

 velocities which have been decided on are used, one 

 after the other. The various fractions are collected 

 in large vessels, and estimated as in the sedimentation 

 method. 



To avoid too high pressures in the piezometer, the 

 " fine gravel " and " coarse sand " may first be separated 

 by means of the 100 meshes to the linear inch sieve, 

 and the portion passing through reserved for elutriation. 

 The operation is stopped when all but the " fine sand " 

 has been removed, and the latter is washed out of the 

 apparatus, evaporated to dryness, and weighed forth- 

 with. 



Mayer gives the following table as representing the products 

 obtained at different pressures, but the apparatus can obviously 



^ For soils containing large quantities of undecomposed lime- 

 stone or dolomite particles, the use of HCl may lead to the 

 inclusion in the silt of many particles which could not be regarded 

 as coming from the soil itself as it exists. The HCl treatment 

 may be recommended unconditionally for alluvial soils. 



