THE PROPERTIES OF SOILS 201 



For the next experiment procure a piece of good clay ; failing 

 any in the neighbourhood, buy a few pounds of modelling clay. 

 Knead a piece up and beat it into a little brick an inch or so 

 square in section and about five inches long, mark two points 

 on the surface exactly ten centimetres apart, and put the brick 

 aside in a warm place to dry. Make up a second brick, but before 

 you knead up the clay incorporate with it as fully as possible a few 

 cubic centimetres of milk of lime, again mark off a length of ten 

 centimetres on the surface. Moisten some of the clay soil used in 

 the previous experiments by adding overnight about 20 per 

 cent, of its weight of water, and with it build up another brick, 

 this time knocking the clay about as little as may be consistent 

 with pressing it firmly together into brick form. When the 

 three bricks have dried it will be found from the relative position 

 of the marks that they have all shrunk considerably, the raw well 

 kneaded clay most of all. Then compare the hardness of the 

 three bricks by breaking them between the fingers ; the soil 

 breaks and crumbles without much difficulty, whereas the clay 

 proper is extremely hard and tough, though the tenacity of that 

 which had been worked up with lime has been much reduced. 

 These then are essential properties of clay, shrinkage on drying 

 and tenacity of the dried mass ; they are linked with its impervi- 

 ousness to water and its plasticity in a wet state, and all depend 

 upon the fineness of grain of the particles making up the clay. 

 Each property is most pronounced when the fineness of grain is 

 fully developed by kneading the clay in a wet state. The particles 

 of a piece of clay that has been subjected for a time to the action 

 of the weather gradually rearrange themselves under the alternate 

 wettings and dryings, freezings and thawings, and unite into 

 loose groups, so that the whole mass simulates a coarser grained 

 material which shrinks less on drying and is then more easily 

 powdered. 



Since compounds of lime act upon the clay in a similar way 

 by causing the finest particles to clot together, it is necessary 

 to examine our soils a little both as to the amount of lime 

 they contain and their behaviour towards that substance. The 

 most universal compound of lime is the carbonate, which exists 

 in a comparatively pure state as chalk or limestone and when 



