42 



ItEPOKT OF CHEMICAL LAliOKATOHY 



'I'echnique of 

 determination 

 of ■■Silt" 



Classification 

 by size 



The beakers are placed in position and, the vent-cock having been closed, the water 

 tap is turned on very cautiously until the water is seen to appear in the glass tubes. 

 If the level rises equally in all, thus indicating that no air is included, the flow- 

 is allowed to continue until the water reaches the bend in the glass tube. At this 

 moment the full force of the water is quickly turned on. If the operation is not 

 carried out as indicated some of the siphons may begin to act too quickly, with the 

 result that their pull on the water may be so great as to prevent entirely the filling of 

 the others. "When the beakers are about half filled, the pressure is greatly reduced so 

 that the level of the water rises very slowly. By operating in this fashion even very 

 considerable differences in the diameters of the tubes and beakers will cause no trouble, 

 since the back pressure in those in which the flow is more rapid acts as a check on the 

 incoming water. If the filling is performed with care there will be an almost absolute 

 equality in the level throughout. 



In order to secure the above equality of level the supply tubes should not be too 

 small. Those in the apparatus in use have a bore of 0-5 cm. If the distributing 

 vessel (A) is made as shown, with the top inclined at a slight angle, and the tubes 

 are cut off flush with- its under-surface, the inclusion of air with the water in the 

 tubes very rarely takes place. The same result may be attained by substituting for 

 the distributing vessel A, a metallic tube tilted at a slight angle. 



During the time the water level is slowly rising in the beakers there is ample 

 opportunity to mix the soil well with the water by stirring with the glass inlet tubes 

 which, as shown in the cut, are connected with the supply tubes by means of flexible 

 rubber tubing. 



At the end of the period fixed upon for subsidence of the larger particles, the vent- 

 cock is opened and the supernatant water siphoned off. When this operation has been 

 completed, the water is again turned on (the vent-cock being still open) and a small 

 amount of water is run through in order to wash out the last of the turbid water. 

 The vent-cock is then closed and the entire operation repeated, as described, as often 

 as may be necessary. 



The Classification of Soil Paeticles 



The want of uniformity in the system of classification of soil particles no less than 

 the diversity in the methods of separation, is greatly to be deplored. Much of the 

 value of the results is lost by reason of the impossibility of comparison with those of other 

 observers. In a paper presented to the International .\gro-geological Congress held at 

 Stockholm in August, 1910, Dr. Atterberg of Kalmar suggested the following limits, with 

 a view to their general adoption : — 



Cuarse sand (unretentive of moisture) ... 

 Fine sand (water-retaining) 



Silt 



Clay ... 



The times of subsidence (in a 10 centimetre 

 of the two series of silts and clays were ; — 



Silts 



Clays 



•03 mm. 

 •02 mm. 



3J minutes 

 7') minutes 



•0U3 mm. 

 •002 mm. 



4 hours 

 8 hours 



