NOTES AND QUERIES. 71 
vary according to many circumstances, and notably according to 
the greater or less exposure of the mass to the air. The outside 
of a heap of leaves has a brown colour, and is undergoing a 
somewhat simple process of slow combustion or decay under the 
action of the oxygen of the air. In the deeper layers the colour 
of the vegetable matter is dark brown or black, and therein 
changes of a more complicated kind, called putrefaction, are 
proceeding. In all these changes the action of living micro- 
organisms takes an important part. The conversion of the dead 
leaf, whose form we can recognise in the dark-coloured formless 
mould, is one in which the organic matter of the leaf is chiefly 
concerned. It is partly oxidised; the carbon being converted 
into carbonic acid, and the hydrogen being converted into water. 
These volatilise and escape into the air, and as they do so the 
layer becomes gradually more and more consolidated. The mineral 
matter remains behind, and becomcs relatively more abundant as 
the volatilisation of organic matter proceeds. Much interest 
attaches to the nitrogenous matter of the leaf, whose fate varies 
much according to circumstances. Some of it decomposes in such 
a way as to give off its nitrogen as free nitrogen gas. Some of 
it escapes as carbonate of ammonia, while the greater part of it 
remains behind as nitric acid, in combination with lime or other 
base. 
The composition of leaf-mould is thus very variable, but some 
notion of its value can be had from knowing the composition of 
the dead leaves from whose decay it is produced. As regards 
nitrogen, the dry dead leaves of some of the more common forest 
trees have on an average about one per cent., viz.:— 
Per cent. Nitrogen. 
Beech leaves, . ; j : : 0:8 
Oak leaves, : i . d : 1-0 
Spruce Fir, 5 : : : ; 13 
Scots Fir, F : : : : i 
Larch, . J : : c : 0:9 
That is a large amount of nitrogen, averaging twice as much as is 
contained in ordinary well-made farmyard manure. That it is 
not so active as farmyard manure is due to its not being sufficiently 
rotted to enable any of the nitrogenous matter to be converted 
into soluble substances, such as ammonia salts or nitrates, but 
