92 THE ORIGIN AND NATURE OF SOILS [chap. 



and retains but a small proportion when thoroughly 

 drained ; puddled clay is quite impervious to water. 



We may make one more experiment with clay : 

 Work up a few grammes of clay with a gallon or so of 

 distilled or rain water, and divide the turbid water that 

 results among several jars. To one add a few grains of 

 salt, to another a few cubic centimetres of lime water, 

 to a third a little sulphate of lime dissolved in water, to 

 a fourth a few drops of ammonia, to a fifth a few drops 

 of acid, and leave a control without addition; put them 

 all aside to settle down undisturbed. The liquid to 

 which nothing has been added will take a long time to 

 clear, with the ammonia it will take longer still, days or 

 even weeks. The acid, on the contrary, causes the 

 solution to clear in a comparatively short time ; the lime 

 water and the sulphate of lime also induce clearing, 

 though not so rapidly, and the salt is still less effective. 

 The appearance of the quickly clearing liquids gives us 

 some clue to what takes place ; the acid and the salts, 

 especially salts of lime, induce the very fine particles of 

 which the clay is composed to clot together and coagulate, 

 or flocculate, as we shall prefer to call it Since each of 

 the compound particles thus formed must be heavier 

 than the units of which it is composed, it falls to the 

 bottom of the liquid more quickly, and generally behaves 

 as though it were something of the order of a grain of 

 sand. Clay particles will also flocculate if left to them- 

 selves and exposed to changes of temperature, wettings 

 and dryings, freezings and thawings, and this fact has a 

 very important bearing on the working of clay soils. 

 On the other hand, any handling or knocking about of 

 the clay soil when in a wet condition breaks down the 

 flocculation and sets all the fine particles free once 

 more, making the clay more characteristically clay than 

 ever. 



