THK MECHANU'Al, ANALYSIS OF AKIl) HOIl.S 43 



In the paper referred to, Dr. Atterberg rather favoured the first series of limits 

 from the point of view of the shortening of the times of subsidence. In a more recent 

 connnunieation, however, he states that against this should be counted the fact that the 

 limit of -002 mm. for clay has already been rather widely adopted and, further, that the 

 limit of "2 mm. for the water-retaining sand is more nearly correct than is that of '3 mm. 

 Our own experience with Sudan soils leads us very greatly to prefer the limits 02 and Limits of 02 

 •002 mm., the latter especially, as it differentiates more sharply the soils with which we ^^^ , "'"'' 



^ " ^ ^ preferred 



have to deal. We have therefore adopted these limits, provisionally, with the hope that a 

 general agreement on these lines may be effected. As already noted the figures given 

 are only approximate, the clay being more accurately defined by the time required 

 for its subsidence. 



Resume of the Method used fok the Mechanical Analysis 



The following is a short description of the method of mechanical analysis as carried 

 out in these laboratories. 



The separation is made, as has been stated, into particles as follows : — 



(a) Stones and tjrarel above 2 mm. 



(6) Fine soil, less than 2 nun. 



The latter is further subdivided into : — 



Coarse sand (non-retentive of water) ... ... 2 mm. to 0'2 mm. 



Fine sand (water-retaining) ... . . . . -2 mm. to '02 mm. 



Silt -02 niiu. to -002 mm. 



Glai/ ... ... ... ... ... ... -002 nmi. and below 



The air-dry soil is well mixed, the larger masses being crushed by means of a 

 wooden rolling-pin. One hundred grammes are weighed out on a rough balance and 

 passed through a sieve with round holes, 2 mm. in diameter. The portion, if any, 

 which remains on the sieve is washed with water to separate the adhering sand and 

 soil, dried on the water-bath and weighed. This constitutes the " stones and gravel." 



The jjortion passing the 2 mm. sieve is passed through a sieve with round holes 

 1 mm. diameter. (The material which passes is taken for the chemical examination 

 and for the further mechanical analysis.) In the above operation the larger masses of 

 soil are treated with the rolling-pin, and also, if not too hard, by rubbing with a cork 

 bung. When most of the soil has been passed through the sieve the remainder is 

 washed on the sieve by a current of water aided by a camel-hair brush, the small 

 quantity of soil washed away being disregarded. 



When a number of soils are being examined at the same time, the residues on the 

 sieves are not treated with water at once, but are put aside, in marked beakers, the 

 washings being performed later. In this way the sieves are kept dry during the operations 

 of sifting the soils. 



The moisture having been removed by several hours' drying in the air-oven at Resume of 

 110 C, a quantity of the fine dry soil is taken, less than 5 grammes in proportion to the technique 

 amount of particles between 2 mm. and 1 mm. known to be present in the original soil. 

 This plan of excluding the larger particles is adopted in order more safely to secure an 

 average sample when the small quantity (less than 5 grammes) is weighed out. 



The weighed portion of fine soil is either puddled by the aid of the brush, as described 

 on page 37 or, if a number of soils are to be treated at the same time, it is placed in a 

 shaker bottle, 100 c.c. of water containing -2 gramme of sodium cai'bonate added, and 

 shaken in the machine, for 2 hours, as described on page 35. The soil and water are then 



