BOOK II. MANURES. 235 



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 (1029.) 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. 



1112. 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 oper- 

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



SUBSECT. 1. The Theory of the Operation of Manures of Animal and Vegetable Origin. 



1113. 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. 



1114. 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 affording solid matters capable of being dissolved by water, or gaseous substances 

 capable of being absorbed by the fluids in the leaves oi" 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. 



1115. Mucilaginous, gelatinous, saccharine, oily, and extractive Jluids, carbonic acid, and 

 ivater, 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. 



1116. 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 afford, 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- ab- 

 sorbed, and carbonic acid formed ; heat will be produced, and elastic fluids, principally car- 

 bonic 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 

 suffered 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 fer- 

 mentation always contains acetic acid ; and when albumen or gluten exists in the vege- 

 table 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 fer- 

 mentation, 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 exposed 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 vegetable substances ; 

 oxygen is absorbed and carbonic acid and ammonia formed in the process of their putre- 

 faction. They produce fetid, compound, elastic fluids and likewise azote : they afford 

 dark-colored acid and oily fluids, and leave a residuum of salts and earths mixed with 

 carbonaceous matter. 



1117. 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 different other acid, saline, and earthy 



, matters. 



1118. General treatment of organic manures. Whenever manures consist principally of 



