312 Mechanical Aufdi/sis of Soils and other Diapermnis 



is delivered into a fiat porcelain dish wliich lias been previously ignited 

 and weighed. Dishes ordinarily used for the estimation of total solids 

 in milk are convenient for the purpose. The sample is taken to dryness 

 on the water bath and, if the estimation is to be made on unignited 

 material, weighed after attaining constant weight. Ordinarily it is 

 ignited in a muffle, an operation which only takes a few minutes at red 

 heat, and weighed after coohng in a desiccator. From the weight of 

 ignited material the concentration of the sample of suspension is calcu- 

 lated. By sampling in such a way that successively smaller values of 

 depth/time are used, the same suspension may be shaken up and sampled 

 over and over again. The partial concentration of any fraction at a 

 given depth is unaltered until the top of the fraction column has sunk 

 below that depth, as will be seen by reference to Fig. 1. The removal 

 of a sample of suspension does not therefore aiiect the concentration of 

 the suspension with respect to fractions of smaller velocities. 



In an actual experiment a 2-5 per cent, suspension of a clay was 

 prepared by shaking up 100 grams of powdered clay for 24 hours with 

 2 litres of water containing 100 c.c. of 1 per cent, sodium carbonate 

 solution and making up finally to 4 litres. A litre cylinder was then 

 filled to within 3-4 era. of the top with the well mixed suspension and 

 after again shaking for a minute, allowed to stand for six minutes. 

 A 20 c.c. sample was then withdrawn at 36 cm. depth as described above. 

 After drying and ignition, the weight of ignited material was found to 

 be -376 gram. Subtracting -005 gram for the amount of sodium car- 

 bonate in 20 c.c, we have the nett weight of ignited material as -371 gram 

 and the concentration of the suspension at the point sampled, 1-855 

 per cent. The original concentration being 2-5 per cent., we find that 

 the concentration of the suspension at 36 cm. after six minutes is 



r855 



•jr~^ X 100=74-2 per cent, of the original concentration. In other 



words 74-2 per cent, of the clay, reckoned as ignited material, has a 

 settbng velocity less than -1 cm. /sec. Other determinations were made 

 for successively smaller velocities and the residts are set out in Table I. 

 For the sake of brevity the w-eight of ignited material is given after 

 subtraction of the -005 gram of .sodium carbonate. 



In Fig. 3 cui-ves are shown for the clay of the experiment just 

 described and for a few other ty})ical clays and soils. The vertical dotted 

 lines, ^I, B, and t', are the ordinates corresponding to clay, fine silt, and 

 silt, respectively, according to their settling velocities in the Enghsh 

 method. In order to bring the fine sand, coarse sand and fiiie gravel 



