32G 
air ; but this, as was mentioned in the introductory 
Lecture, is not a correct statement. Oxygene is 
absorbed by the vegetable film, and, perhaps, in 
certain cases, azote ; but the earths, the great ele- 
ments of soils, cannot be combined with new ele- 
ments from the air ; none of them unite to azote ; 
and such of them as are capable of attracting car- 
bonic acid, are always saturated with it in those 
soils on which the practice of fallowing is adopted. 
The vague ancient opinion of the use of nitre, and 
of nitrous salts in vegetation, seems to have been 
one of the principal speculative reasons for the 
defence of summer-fallows. Nitrous salts are pro- 
duced during the exposure of soils containing 
vegetable and animal remains, and in greatest abun- 
dance in hot weather ; but it is probably by the 
combination of azote from these remains with oxy- 
gene in the atmosphere that the acid is formed ; 
and at the expense of an element, which otherwise 
would have formed ammonia ; the compounds of 
which, as is evident from what was stated in the 
last Lecture, are much more efficacious than the 
nitrous compounds in assisting vegetation. 
When weeds are buried in the soil, by their ab- 
sorption of oxygene and gradual decomposition, 
they furnish a certain quantity of soluble matter ; 
and a soil will certainly in consequence produce 
better crops at the end of a fallow; but the use of 
this practice must depend upon the quantity of ve- 
getable fibre and its nature, and upon the quality of 
the soil. Carbonic acid gas is formed during the 
whole time by the action of the vegetable matter 
upon the oxygene of the air, and the greater part 
