OF MOULD. 15 



earth, and water tliat come in contact with them ; it is 

 necessary, then, that they should find immediately around 

 them the nutritive principles requisite for their growth, 

 and for the exercise of their functions; it is necessary 

 that they should be able to extend their roots, in order to 

 draw from the soil its nourishing juices; and to fasten 

 themselves in the earth, so as to be secure from being 

 dried up by heat or uprooted by the winds. 



As all the qualities required by a vigorous vegetation 

 cannot always be found united in land appropriated to cul- 

 tivation, we are led to examine the nature of earths, and 

 the differences which exist amonir them. 



ARTICLE I. 



Of Mould. 



All plants, when dead, are more or less readily decom- 

 posed ; and in undergoing these changes, which are 

 greatly facilitated by air and heat, they form products 

 with which it is of importance for us to be acquainted ; 

 as the principal aliments of living plants are furnished by 

 those that are dead. Decomposition is most rapid in suc- 

 culent vegetables, and in those which are collected in 

 heaps ; but a high degree of atmospheric temperature and 

 the humidity of plants contribute powerfully to accelerate 

 it. During decomposition much carbonic acid is given 

 out ; a part of this exists in combination with the constitu- 

 ent principles of the plant, and a part of it is produced by 

 the action of the oxygen of the atmosphere upon the car- 

 bon of the plant ; hydrogen, which is probably furnished 

 by the decomposition of the watery particles, and is 

 generally carburetted, is likewise exhaled, as also am- 

 moniacal gas when its elements exist in the plant* 

 When large masses of vegetables are in a state of fermen- 

 tation, heat is always produced ; but if they have been 

 dried, it is necessary to collect them into heaps, and 

 moisten them slightly in order to determine their fermenta- 

 tion and decomposition ; in this case the heat produced 

 is sometimes so great as to cause the combustion of the 

 mass ; a phenomenon which occurs when hay is stacked 

 without being sufficiently dry, or when ropes, hemp, or flax 

 are piled up wet. 



