328 SCIENCE OF AGRICULTURE. Part 11. 



peculiar organisation is contrived, and upon which its healthy existence depends. The 

 elementary bodies above enumerated are all contained in animal, and the three first in 

 vegetable matters. Sometimes vegetables, though very seldom, contain a small quantity 

 of nitrogen. As certain salts are also constantly found to be present in healthy living 

 vegetables, manures or vegetable food may, consequently, be distinguished into animal, 

 vegetable, and saline. The authors whom we have already mentioned (2065.) as produc- 

 ing the first chemical treatises on soils, were also the first to treat chemically of manures. 

 Of these, the latest in the order of time is Sir H. Davy, from whose highly satisfactory 

 work we shall extract the greater part of this chapter. 



Sect. I. Of Manures of Animal and Vegetable Origin. 



2162. Decaying animal and vegetable substances constitute by far the most important 

 class of manures, or vegetable food, and may be considered as to the theory of their 

 operation, their specific kinds, and their preservation and application in practice. 



SuBSECT. 1. T/ie Theory of the Operation of Manures of Animal and Vegetable Origin. 



2163. The rationale of organic manures is very satisfactorily given by Sir H. Davy, 

 who, after having proved that no solid substances can enter in that state into the plant, 

 explains the manner in which nourishment is derived from vegetable and animal sub- 

 stances. 



2164. Vegetable and animal substances deposited in the soil, as is shown by universal ex- 

 perience, are consumed during the process of vegetation ; and they can only nourish the 

 plant by aflTording solid matters capable of being dissolved by water, or gaseous substances 

 capable of being absorbed by the fluids in the leaves of vegetables ; but such parts of 

 them as are rendered gaseous, and pass into the atmosphere, must produce a comparatively 

 small effect, for gases soon become diffused through the mass of the surrounding air. 

 The great object, therefore, in the application of manure should be to make it afford as 

 much soluble matter as possible to the roots of the plant ; and that in a slow and gra- 

 dual manner, so that it may be entirely consumed in forming its sap and organised 

 parts. 



2165. Mucilaginous, gelatinous, saccharine, oily, and extractive fluids, carbonic 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 ex- 

 cess of fibrous and insoluble matter, which must undergo chemical changes before they 

 can become the food of plants. 



2166. The nature of the changes on these substances; of the causes which occasion them, 

 and which accelerate or retard them ; and of the products they aflTord, have been scientifi- 

 cally stated and explained by our great agricultural chemist. If any fresh vegetable matter 

 which contains sugar, mucilage, starch, or other of the vegetable compounds soluble in 

 water, be moistened, and exposed to air, at a temperature from 55 to 80, oxygen will 

 soon be absorbed, and carbonic acid formed ; heat will be produced, and elastic fluids, 

 principally carbonic acid, gaseous oxide of carbon, and hydro-carbonate will be evolved ; 

 a dark-colored liquid, of a slightly sour or bitter taste, will likewise be formed; and if 

 the process be suflTered to continue for a time sufficiently long, nothing solid will remain, 

 except earthy and saline matter, colored black by charcoal. The dark-colored fluid 

 formed in the fermentation always contains acetic acid ; and when albumen or gluten 

 exists in the vegetable substance, it likewise contains volatile alkali. In proportion as 

 there is more gluten, albumen, or matters soluble in water, in the vegetable substances 

 exposed to fermentation, so in proportion, all other circumstances being equal, will the 

 process be more rapid. Pure woody fibre alone undergoes a change very slowly ; but 

 its texture is broken down, and it is easily resolved into new aliments, when mixed with 

 substances more liable to change, containing more oxygen and hydrogen. Volatile and 

 fixed oils, resins, and wax, are more susceptible of change than woody fibre, when ex- 

 posed to air and water ; but much less liable than the other vegetable compounds ; and 

 even the most inflammable substances, by the absorption of oxygen, become gradually 

 soluble in water. Animal matters in general are more liable to decompose than veget- 

 able substances; oxygen is absorbed and carbonic acid and ammonia formed in the 

 process of their putrefaction. They produce fetid, compound, elastic fluids and like- 

 wise azote : they afford dark-colored acid and oily fluids, and leave a residuum of salts 

 and earths mixed with carbonaceous matter. 



2167. The principal animal substances which constitute their diflferent 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 different other acid, saline, 

 and earthy matters. 



2168. General treatment if organic manures. Whenever manures consist principally of 

 matter soluble in water, it is evident that their fermentation or putrefaction should be 



