The part that hums away 



35 



more closely. We will take some leaf mould, some black 

 mould from a hollow in the bank, some from a tree, soils 

 from a wood, a well-manured garden, a field and some 

 subsoil. All except the subsoil have a dark colour, but 

 the wood and garden soils are probably darker than the 

 field soil Now weigh out 2 grams of each of these 

 and heat in a dish as you did the soil on p. 4 ; notice 

 that all except the subsoil go black and then begin to 

 smoulder, but the moulds smoulder more than the soils. 

 Then weigh again and calculate how much has burnt 

 away in each case. Here are some results that have 

 been obtained at Harpenden : — 



The mould nearly all burns away and its dark colour 

 entirely goes, so also does the dark colour of the soil. 



Our supposition explains why, in the case of soils, the 

 less the blackness, the less the loss on burning. If the 



1 The top two inches of soil only were collected here, and there were 

 many leaves, twigs, etc. mixed in. Soils from different woods vary 

 considerably. If the sample is taken to a greater depth the loss on 

 burning is much less, and may be only 5 or 6 per cent. 



8—2 



