11 
afford by analysis ; and though taste and appear- 
ance must influence the consumption of all articles 
in years of plenty, yet they are less attended to in 
times of scarcity, and on such occasions this kind 
of knowledge may be of the greatest importance. 
Sugar and farina or starch, are very similar in com- 
position, and are capable of being converted into 
each other by simple chemical processes. In the 
discussion of their relations, I shall detail to you 
the results of some recent experiments, which will 
be found possessed of applications both to the 
economy of vegetation, and to some important pro- 
cesses of manufacture. 
All the varieties of substances found in plants, 
are produced from the sap, and the sap of plants is 
derived from water, or from the fluids of the soil, 
and it is altered by, or combined with principles 
derived from the atmosphere. The influence of 
the soil, of water, and of air, will therefore be the 
next subject of consideration. Soils in all cases 
consist of a mixture of different finely divided 
earthy matters ; with animal or vegetable sub- 
stances in a state of decomposition, and certain 
saline ingredients. The earthy matters are the 
true basis of the soil ; the other parts, whether 
natural, or artificially introduced, operate in the 
same manner as manures. Four earths generally 
abound in soils, the aluminous, the siliceous, the 
calcareous, and the magnesian. These earths, as 
I have discovered, consist of highly inflammable 
metals united to pure air or oxygene ; and they 
are not, as far as we know, decomposed or altered 
in vegetation. 
