NATURE AND ORIGIN OF TILTH 39 



of ultimate particles, a state in which it exhibits its maximum 

 degree of cohesion. 



The action of mechanical force in increasing the plasticity 

 and cohesion of clay is well known to the brickmaker and 

 potter ; no clay is used by them till it has gone through the 

 pug-mill, and been ground and beaten in a most thorough 

 manner. If an engineer desires to make clay impervious to 

 water, the same system of mechanical working while it is 

 in a wet state is pursued ; the clay is then said to be puddled. 

 The farmer is equally well acquainted with the effects of 

 mechanical force on wet clay. Ploughing a clay soil in wet 

 weather is known to be disastrous to its condition. The 

 effect of very heavy rain on clay land is also to destroy the 

 surface tilth. In all these cases the work done consists in 

 the destruction of compound particles, and the resolution 

 of the clay into a mass of ultimate particles. 



Under what conditions are compound particles formed? 

 They arise spontaneously under certain natural conditions. 

 If the lump of beaten loam mentioned above is thrown out 

 upon a garden bed, and left exposed to variable weather for 

 a few months, it will be found completely altered. The lump 

 will be found to have increased considerably in bulk, and, 

 at last, when partially dry, it will fall to pieces when touched. 

 It is now said to have become ' mellow ' ; the condition of 

 tilth has been re-established; it is now once more a mass 

 of large compound particles having only a moderate adhesion 

 to each other. This change has been brought about by 

 natural agents without the aid of tillage. 



The conversion of ultimate into compound particles will 

 not take place in a dry soil, nor in a very wet one. The 

 soil is in the most favourable condition for producing 



