LECTURE VIII. 



How the supply of food for plants is kept up in the general vegetation of the globe. — Propor- 

 tion of their fo«il drawn by plautsfrom the air.— Supply of carbonic acid.—Supply of ammo- 

 nia and nitric acid.— Production of both in nature.— Theory of their action on living vege- 

 tables. — Concluding observations. 



Having shown in the preceding Lecture in what way, and by what 

 chemical changes, the substances of which plants chiefly consist naay 

 be produced from those on which they live, — there remains only one 

 further subject of inquiry in connection with the organic constituents of 

 plants. 



Plants, as we have already seen, derive much of their sustenance from 

 the carbonic acid of the atmosphere ; yet of this gas the air contains only 

 a very small fraction, and in so far as experiments have yet gone, this 

 fractional quantity does not appear to diminish — how, then, is the sup- 

 ply of carbonic acid kept up ? 



Again, plants most probably obtain much of their nitrogen either from 

 ammonia or from nitric acid ; and yet, neither in the soil nor in the air 

 do these compounds permanently exist in any notable quantity, — whence 

 then is the supply of these substances brought within the reach of plants ? 



The importance of these two questions will appear more distinctly, if 

 we endeavour to estimate how much of their carbon plants really draw 

 from the atmosphere — and how much of the nitsogen they contain must 

 be derived from sources not hitherto pointed out. 



§ I. Of the proportion of their carhon which plants derive from the 

 atmosphere. 



On this subject it is perhaps impossible to obtain perfectly accurate 

 results. Several series of experiments, however, have been published; 

 which enable us to arrive at very useful approximations in regard to the 

 proportion of their carbon which plants, growing in a soil of ordinary 

 fertility, and in such a climate as that of Great Britain, actually extract 

 from the air by which they are surrounded. 



1°. In an experiment made in 1824, upon common borage (Borago 

 officinalis), Lampadius found that after a growth of five months (from 

 the 3rd of April to the 6th of September) this plant produced ten times 

 as much vegetable matter as the soil in which it grew had lost during 

 the same period.* In other 'woj:(\Sy it had drawn nine-tenths of its car- 

 hon from the air. 



2°. The experiments of Boussingault were made, if not with more 

 care, at least upon a greater number of plants, and were protracted 

 through a much longer period. It is necessary that we should under- 

 stand the principle on which they were conducted, in order that we 

 may be prepared to place confidence in the determinations at which he 

 arrived. 



* The above experiment may have been correctly made, but the result appears at first 

 siglit too startling to be readily received as indicative of the proportion of their sustenance 

 drawn by plants from the air in the general vegetation ofthp, globe. 



