THE BURNING LOG. 111 
carried through the system of the tree it under- 
goes various changes to fit it for the different 
offices it has to fulfil. Wood, for instance, is not 
purely carbon, but consists of carbon and the 
elements of water, which were most probably 
obtained by the tree from the decomposition of 
the water taken up by the roots. 
To pause for a moment here and to look back; 
asking ourselves What have we seen as to the food 
of plants? The answer is very simple: They live 
upon carbonic acid, and ammonia, and rain,—all 
derived from the air. Yet a little fireside experi- 
ment will suffice to convince us, that this is not 
the whole of the food of plants. <A_ brightly 
burning log of wood lies blazing and crackling 
on the top of the fire. Burning away in showers 
of sparks, and casting up pale lambent flames, and 
‘now and then informing us that its cells contain 
water, which, being converted into steam and ex- 
ploding, causes the cheerful crackling sounds we 
hear—the fire gradually dies down, we retire to 
rest, and repairing to the fire-place early in the 
following morning, we discover nothing left of the 
log but a heap of soft white ashes. Why has it 
‘not burnt away altogether, as, if the wood had 
