Book III. OPERATION OF MINERAL MANURES. 343 



organised substances in the free atmosphere are noxious processes; beneath the surface 

 of the ground, they are salutary operations. In this case the food of plants is prepared 

 where it can be used ; and that which would offend the senses and injure the health, if 

 exposed, is converted by gradual processes into forms of beauty and of usefulness ; the 

 fetid gas is rendered a constituent of the aroma of the flower, and what might be poison 

 becomes nourishment to animals and to man. 



2278. To preserve dung for any time, the situation in which it is kept is of importance. 

 It should, if possible, be defended from the sun. To preserve it under sheds would be 

 of great use; or to make the site of a dunghill on the north side of a wall. The floor 

 on which the dung is heaped should, if possible, be paved with flat stones; and there 

 should be a little inclination from each side towards the centre, in which there should be 

 drains connected with a small well, furnished with a pump, by which any fluid matter 

 may be collected for the use of the land. It too often happens that a dense mucilaginous 

 and extractive fluid is suffered to drain away from the dunghill, so as to be entirely lost 

 to the farm. 



Sect. II. Of Manures of Mineral Origin* 



2279. Earthy and saline manures are probably of more recent invention, and doubtless 

 of more uncertain use, than those of animal and vegetable origin. The conversion into 

 original forms of matter which has belonged to living structures, is a process that can be 

 easily understood ; but it is more difficult to follow those operations by which earthy and 

 saline matters are consolidated in the fibre of plants, and by which they are made subser- 

 vient to their functions. These are capable of being materially elucidated by modern 

 chemistry ; and shall here be considered as to the theory of their operation and as to their 

 specific kinds. 



Subsect. 1. Theory of the Operation of Mineral Manures. 



2280. Saline and calcareous substances form the principal fossil manures. Much has 

 been written on lime and common salt, both in the way of speculation and reasoning 

 from facts, which, from want of chemical knowledge, has turned to no useful account, 

 and cultivators till very lately contented themselves with stating that these substances 

 acted as stimuli to the soil, something like condiments to the digestive organs of animals. 

 Even chemists themselves are not yet unanimous in all their opinions ; but still the result 

 of their enquiries will be found of great benefit to the scientific cultivator. 



2281. Various opinions exist as to the rationale of the operation of mineral manures. 

 " Some enquirers," Sir H. Davy observes, " adopting that sublime generalisation of the 

 ancient philosophers, that matter is the same in essence, and that the different substances, 

 considered as elements by chemists, are merely different arrangements of the same inde- 

 structible particles, have endeavoured to prove, that all the varieties of the principles 

 found in plants, may be formed from the substances in the atmosphere ; and that vege- 

 table life is a process in which bodies, that the analytical philosopher is unable to change 

 or to form, are constantly composed and decomposed. But the general results of expe- 

 riments are very much opposed to the idea of the composition of the earths, by plants, 

 from any of the elements found in the atmosphere, or in water, and there are 

 various facts contradictory to the idea." Jacquin states, that the ashes of glass- wort 

 (Salsola Soda), when it grows in inland situations, afford the vegetable alkali; when it 

 grows on the sea-shore, where compounds which afford the fossil or marine alkali are 

 more abundant, it yields that substance. Du Hamel found that plants which usually 

 grow on the sea-shore made small progress when planted in soils containing little com- 

 mon salt. The sun-flower, when growing in lands containing no nitre, does not afford 

 that substance ; though when watered by a solution of nitre it yields nitre abundantly. 

 The tables of De Saussure show that the ashes of plants are similar in constitution to the 

 soils in which they have vegetated. De Saussure made plants grow in solutions of dif- 

 ferent salts ; and he ascertained that, in all cases, certain portions of the salts were 

 absorbed by the plants, and found unaltered in their organs. Even animals do not 

 appear to possess the power of forming the alkaline and earthy substances. Dr. Fordyce 

 found that when canary birds, at the time they were laying eggs, were deprived of access 

 to carbonate of lime, their eggs had soft shells ; and if there is any process for which 

 nature may be conceived most likely to supply resources of this kind, it is that connected 

 with the reproduction of the species. 



2282. It seems a fair conclusion, as the evidence on the subject now stands, that the dif- 

 ferent earths and saline substances found in the organs of plants, are supplied by the soils 

 in which they grow ; and in no cases composed by new arrangements of the elements in 

 air or water. What may be our ultimate view of the laws of chemistry, or how far our 

 ideas of elementary principles may be simplified, it is impossible to say. We can only 

 reason from facts. We cannot imitate the powers of composition belonging to vegetable 

 structures ; but at least we can understand them ■ and as far as our researches have gone, 



