253 
ON MANURES AND COMPOSTS. 
The pure or simple earths alone do not perform any material part in the pro- 
cess of vegetable nutrition ; they may be considered as the media by which the 
plant is supported, and through which it is enabled to supply itself with the aliment 
necessary to the g-rowth and development of its parts. 
That aliment appears to be furnished either by decayed vegetation naturally 
(^^ e. by absorption of the products of natural fermentation, either from the surface 
of the g-round, or floating- in the atmosphere), or artificially, and chiefly by the 
organic materials applied by labour. The earths so supplied with organic matter 
are called soils, and are said to be manured ; and it now remains to examine the na- 
ture and composition of various composts, which experience has proved to possess 
properties fitted to recruit the soil with matter of which it has been deprived by the 
crops that it has borne. 
a Vegetable and animal substances deposited in the soil, as is shown by universal 
experience, are consumed during the process of vegetation ; and they can only 
nourish the plant by affording solid matter capable of being dissolved by water, 
or gaseous substances capable of being absorbed by the fluids in the leaves of 
vegetables. Mucilaginous, gelatinous, saccharine, oily, and extractive fluids, car- 
bonic acid, and water, are substances that, in their unchanged states, contain almost 
all the principles necessary for the life of plants ; but there are few cases in which 
they can be applied as manures in their pure forms ; and vegetable manures in 
general contain a great excess of fibrous and insoluble matter, which must undergo 
chemical changes before they can become the food of plants." " If any fresh 
vegetable matter which contains sugar, mucilage, starch, or other vegetable com- 
pounds, soluble in water, be moistened, and exposed to the air, at a temperature 
of from 55^ to 80*^, oxygen will soon be absorbed, and carbonic acid formed ; heat 
will be produced, and elastic fluids, principally carbonic acid, gaseous oxyde of 
carbon, and hydro-carbonic gas, will be evolved ; a dark-coloured liquid, of a sour 
or bitter taste, will likewise be formed ; and if the process be sufi'ered to continue 
for a time sufl5ciently long, nothing solid will remain, except earthy and saline 
matter, coloured black by charcoal." 
"Animal matters in general are more liable to decompose than vegetable sub- 
stances : oxygen is absorbed, and carbonic acid and ammonia formed in the process 
of their putrefaction. They produce fetid, compound, elastic fluids, and likewise 
azote. They afford dark-coloured acid and oily fluids, and leave a residuum of salts 
and earths mixed with carbonaceous matter. The principal animal substances 
which constitute their different parts, or which are found in their blood, their 
secretions, or their excrements, are, gelatine, fibrine, mucus, fatty or oily matter, 
albumen, urea, uric acid, and other acids, saline and earthy matters." 
" Whenever manures consist principally of matter soluble in water, their fer- 
mentation and putrefaction should be prevented as much as possible. To prevent 
