v.] PROPERTIES OF CLA Y 91 



sand will suffice, while for the clay a piece of pure tile 

 or brick clay before it has been mixed with sand is 

 required, or better still, a sample of modelling clay ; some 

 will be wanted in its natural moist condition, while a 

 little should be dried and roughly powdered. Begin by 

 taking some moist clay and some wetted sand ; the clay 

 can be moulded into shape and will hold together even 

 when squeezed into thin sheets, the sand possesses little 

 or no cohesion. On drying, the sand falls to pieces, the 

 clay retains its shape and becomes hard. Mould a little 

 brick of the clay, 6 to 7 inches long and about i inch 

 square, make two marks on the face exactly 5 inches 

 apart ; put the clay aside to dry, and then measure the 

 distance between the marks ; the clay shrinks in drying. 

 Clay is plastic and coherent when wet, retains its shape 

 on drying, but shrinks and becomes very hard, none of 

 which properties are shared by the sand. Take three 

 funnels, plug their stems loosely with cotton-wool, and 

 weigh out into one 50 grammes of sand, into each of the 

 others 50 grammes of the dry powdered clay ; stand each 

 above a beaker, and pour on to the soils 100 c.c of water. 

 On to one of the clay soils, however, pour a little water 

 to begin with and make it into a paste with the clay, 

 adding water little by little until all the clay is pulped 

 up and then pouring on what remains. Now note both 

 how much water runs through and how long it takes to 

 drain in each case. The water runs through the sand in 

 a few minutes, and when measured the sand will probably 

 not have retained more than 10 c.c. of the water. Through 

 the powdered clay the water may take an hour or more to 

 pass, and when the drainage is complete the clay may 

 have retained almost its own weight of water. In the 

 third funnel, where the clay has been pulped up with 

 water or "puddled," no water at all will have passed. 

 As compared with clay, the sand passes water quickly 



