Agricultural Chemistry. 295 



the quantity of nitro_2:en which the soil receives in the shape of 

 manure, is but a small fraction of the sum of nitrogen which is 

 reaped in the crops ; on the contrary, cultivation on the small 

 scale shows, that the quantity of nitrogen, which is reaped 

 in the crops on a soil richly manured with ammoniacal salts, is 

 but a small fraction of that which has been supplied to the soil. 

 In cultivation on the great scale much more nitrogen, in all 

 experiments with ammoniacal salts on the small scale much less 

 nitrogen, is reaped in the crops than the soil has received in the 

 manure. The considerations on which my explanation is founded 

 are the following: — 



If we imagine to ourselves a lake containing an inexhaustible 

 amount of water, from which hundreds of canals conve}', each a 

 limited quantity of water to as many mills, it is plain that, even 

 if each mill receives the same amount of water, the effect of this 

 water, which is produced by its fall, may be very unequal. One 

 mill grinds, in 24 hours, 20 sacks of flour; another yields in the 

 same time 30, a third 50, a fourth 100 sacks of flour. These 

 unequal effects, with equal quantities of water, depend, as is well 

 known (the fall being supposed tlie same in all), on the con- 

 struction of the mill-wheel. With a badly-constructed wheel, 

 one-half or one-third of the water runs past the buckets without 

 producing any effect. The maximum of effect is produced when 

 ■every drop of water is allowed to exert its proper effect, when all 

 obstacles are removed, which cause a loss of water or interfere 

 with its action ; and this every miller, who understands some- 

 thing of mechanics, can secure by a certain form and construction 

 of the wheel and the buckets. 



Precisely similar is the relation of the atmosphere to plants. 

 The air (and, as I have shown by my analyses, the soil) contains 

 ^n inexhaustible magazine of ainnionia and carl)onic acid. To 

 each field, that is, to each equal surface, is supplied an equal but 

 limited quantity, sufficient for the most luxuriant vegetation ; and 

 the art of the farmer consists essentially in fixing in his fields 

 the whole supply of carbonic acid and ammonia thus offered, or 

 in converting it into a maximum of bread and flesh. This is 

 done in the cultivation of his crops. 



The food of ])lants is taken up bv means of their roots and 

 leaves (see the chapter on the origin and action of humus, ' Agri- 

 cultural Chemistry,' p. 29). " The size lokich a plant acquires in 

 ■a r/iven time is proportional to the surface of the orfjans destined to 

 convey food to it" (p. 81). With the surface and number of the 

 leaves and of the root'fiijres, tlie power of the j)lant to take up 

 ■ammonia and carbonic acid increases, and in the same degree. 

 A plant with ten leaves and ten root-fibres takes uj) in the same 

 time only half as much as a plant with twenty leaves and twenty 



