ORGANS OF DIGESTION. 91 



lakes, and still better than that of springs and pumps ; 

 and why watering from a garden pot with a very finely 

 perforated rose is more advantageous than from one 

 with coarser holes. 



Hence also we discover the principles of manuring, 

 the fresh dung of animals being bad till it is fermented 

 and rotted, in order to form humic acid, which is also 

 accomplished by ploughing in green vegetable crops, 

 by burning the soil, by scattering ashes or quicklime, 

 or mixing the latter in compost heaps. The mere 

 mixture of soils and frequent ploughings proposed by 

 Kretschmar, by Tull, and lately by Mr. E. Lance, 

 therefore do not succeed, as might have been foreseen ; 

 for these will not supply the gases wanted. 



It is on the same principle, that soil overshadowed 

 with thick leaves is good, from its attracting carbonic 

 acid vapours, and preventing their escape. Hence a 

 crop of potatoes, turnips, or vetches fertilises, while 

 corn and flax exhaust, the soil, and hence corn thrives 

 better when clover is sown with it. On this, and on 

 the fact of plants throwing out excrementitious matter 

 into the soil, is founded the practice of the rotation of 

 crops 1 . Volcanic and basaltic countries, from the soil 

 containing much carbonic acid gas, are always very 

 fertile, even where the earth is very shallow, as is 

 every where seen in the vineyards on the Rhine, and 

 in Sicily, and near Naples. The basaltic carses of 

 Scotland are famous for fertility. 



(I) This subject will be more fully explained in the ALPHABET 

 OF SCIENTIFIC AGRICULTURE, now in preparation. 



