DR. SCULEIDEN'S THEORY OF AGRICULTURE. 159 



Flint is so universal that, however necessary, it scarcely ever 

 requires to be supplied artificially, and if so the point would be 

 to apply it in a soluble form. It must be remembered that the 

 greater part of the silex comes back to the land with the manure, 

 while a proportionately large quantity of the more valuable con- 

 stituents is carried off with the grain, a circumstance of great 

 consequence in estimates of cropping and manure, as the relative 

 proportions on which so much depends are totally deranged. 



It does not follow, though under certain methods of farming 

 more inorganic matter comes back continually tliau has been 

 taken away, that inorganic matters are not the important consti- 

 tuents in the nutriment of plants, because, if the alkalies in the 

 harvest be as 2*3, and that in the manure as 2-17, or on the 

 addition of turf ashes, as practised by Boussingault, as 2 "30, 

 the conditions are altogether different, a fact which seems to 

 be too often overlooked. 



A very few words on organic manures must suffice, as so much 

 has already been said on the subject. Nitrogen is received from 

 dung in the form of ammonia. It is a common notion that the 

 proportion of ammonia contained in manure is of great value as 

 regards the nourishment of plants, which is sometimes regarded 

 as the only source from whence tlie nitrogen contained in plants 

 is derived. The great produce of water-meadows, which receive 

 no dung, is sufficient to show that this prejudice is not well 

 founded. Peruvian guano containing 29 * 88 lbs. of nitrogen 

 has almost the same effect as bone-jelly containing 82' 55, and 

 one-third more than sulphate of ammonia containing 50 'To. 

 The substances which have most effect as manures are those 

 which are rich in phosphates, and in a series of experiments the 

 greatest effect was protluced by Flemish dung, consisting of urine 

 and pure human excrement, which is rich in alkalies ; and that 

 these ought to be taken into consideration is proved by tiie 

 favourable effects of Cliili saltpetre (nitrate of soda). It is re- 

 markable too that bone-ashes, rich in phosjihate of lime, produce 

 no effect by themselves, but, when combined with ammoniacal 

 water from gas-works, their effect is surprising, a result wiiich 

 probably depends on the dissolving power of the ammonia on 

 other substances, and especially on phosphates. 



On the whole then it appears that, except in cases of an inex- 

 haustible depth of soil, cultivation cannot succeed, or at least the 

 fertility of the land be fully sustained, from the resources of the 

 soil itself; either a portion must be in meadow, or additional 

 manure must in some shape or other be introduced. 



B. Manures act also on the physical constitution of the ground. 

 The power of absorbing steam and gas from the atmospliere 

 depends almost exclusively on the quantity of clay or humus. 

 The great pohit undoubtedly is to keep up the supply of the 



