23.5 
" at the expense of the oxygen of the air, surrounds 
" every particle of decaying humus. An atmosphere of car- 
" bonic acid is therefore contained in every fertile soil, and 
" is the first and most important food of the young plants 
" lohich grow in it'' Professor Johnston, after saying in 
his third lecture that you do not add carbonaceous matter, 
which you are afterwards to reap back in the form of hay, 
corn, &c., commences his fourth lecture by saying, It being 
proved that plants derive their carbon from the air, the 
*' next inquiry was, in what form did this carbon, which 
" formed so large a proportion of these substances, enter 
" into the circulation of plants? In the first place, it was 
" derived from the carbonic acid of the atmosphere entering 
" into them by the leaves, and in the second place, water 
" trickling through the soil and over the surface of the earth, 
" imbibing a considerable quantity of carbonic acid, and 
" carrying it into plants by their roots so that by this 
account, manure in the shape of carbonic acid, does enter by 
the roots, and is reaped back in the form of hay, corn, &c. 
The learned Professor asks, " Do grasses and trees derive 
" their carbon from the soil ? Then how by their growth 
" do they increase the quantity of carbonaceous matter 
" which the soil contains ?" The answer to this is, that 
grass increases the carbon because it is ploughed up ; and 
the amount of carbon contained in the swarth is thus added 
to the soil ; and that trees increase the quantity of carbon in 
the soil by the addition of carbonaceous matter contained in 
the stumps and roots left when they are cut down. 
But he says, " on this point the rapid growth of peat may 
" be considered absolutely conclusive. A tree falls across 
" a little running stream, dams up the water, and produces 
" a marshy spot. Rushes and reeds spring up, and mosses 
" take root and grow. Year after year new shoots are sent 
* Durham Advertiser, l-2th March, 1841. 
