44 AGRICULTURE 



Then, again, it must not be forgotten that 

 there is an enormous amount of heat in the 

 centre of the earth, proof of which is furnished 

 by hot springs, and active volcanoes, and 

 also by the fact that as one goes down the 

 shaft of a mine the temperature rises steadily 

 and fairly rapidly. On account of the 

 earth's crust being a very bad conductor, 

 the heat present in the centre of the earth 

 is conducted outwards very slowly, but 

 that the internal supply of heat in the 

 earth has to be reckoned with cannot be 

 doubted. 



One of the most important properties of 

 soil is concerned with its power of retaining, 

 absorbing, or fixing plant food from solutions. 

 This property is something entirely different 

 from the power which soil, in common 

 with many other substances, possesses of 

 mechanically filtering out materials held in 

 suspension in water. When a substance is 

 in solution it is quite inseparable by mechani- 

 cal processes of filtration, but it is known 

 that soil possesses the power of removing 

 certain of these substances and of retaining 

 them in its mass. If, for instance, one were 

 to fill with soil a series of glass tubes open at 

 each end, having previously closed one end 

 with wire gauze or muslin, and if certain 



