r 



170 THICKNESS OF THE MOULD Chap. UI. 



nearly so compact a mass as vegetable mould, 

 though each separate particle was very 

 compact. Yet mould is far from being com- 

 pact, as is shown by the number of air- 

 bubbles which rise up when the surface is 

 flooded with water. It is moreover pene- 

 trated by many fine roots. To ascertain ap- 

 proximately by how much ordinary vegetable 

 mould would be increased in bulk by being 

 broken up into small particles and then dried, 

 a thin oblong block of somewhat argillaceous 

 mould (with the turf pared ofi") was measured 

 before being broken up, was well dried and 

 again measured. The drying caused it to 

 shrink by ^ of its original bulk, judging from 

 exterior measurements alone. It was then 

 triturated and partly reduced to powder, in the 

 same manner as the castings had been treated, 

 and its bulk now exceeded (notwithstanding 

 shrinkage from drying) by -^ that of the 

 original block of damp mould. Therefore the 

 above calculated thickness of the layer, formed 

 by the castings from the Terrace, after being 

 damped and spread over a square yard, would 

 have to be reduced by ■^; and this will 

 reduce the layer to "09 of an inch, so that a 



