PROPERTIES OF CLA Y 83 



62. Shrinkage of Clay on Drying, 



A small brick of modelling clay is made, of 6 inches 

 in length. Two marks, exactly 5 inches apart, are made 

 on the clay, and the brick weighed. As the brick dries 

 the distance apart of the two marks is measured from 

 time to time, and the loss in weight determined after 

 each measurement. Eventually drying is completed 

 in the oven, a final measurement and weighing made, 

 and curves drawn to show the connection between 

 shrinkage and water-content. 



The experiment should be repeated with a brick 

 composed of clay mixed with about \ per cent, of quick- 

 lime, and the results compared with those previously 

 obtained. 



CHAPTER VIII 

 Mechanical Analysis of Soil 



The mechanical analysis of a soil consists of its division into 

 various grades of particles according to size. This is a most 

 important point with regard to the physical properties of a soil, 

 especially as a measure of the very fine particles present. The 

 latter give the soil power to retain moisture and resist drought, 

 whilst shrinkage on drying and tenacity when dry are largely 

 dependent on low proportions of humus, sand, and chalk. 



Two types of method are open for the mechanical soil-analysis, 

 viz. : {a) Those depending on sedimentation in still water and 

 decantation after a time ; {b) Hydraulic methods, in which 

 separation is effected by water-currents of varying speeds (elutria- 

 tion). The former method is the one generally adopted in this 

 country, although elutriation offers a great deal of scope in its 

 application. 



63. Sampling Soil for Analysis, 



Soil samples are generally taken to a depth where 

 soil begins to merge in subsoil, or, if this is not a 



