ON THE FOOD OF PLANTS. 107 



" In truth, the fertilizing power of putrid 

 " animal and vegetable substances were pretty 

 " fully known even in the remotest ages ; but 

 " most speculatists have hitherto attributed them 

 " to the oleaginous, mucilaginous, or saline par- 

 " tides then developed ; forgetting that land is 

 " fertilized by paring and burning, though the 

 " oleaginous and mucilaginous particles, are 

 " thereby consumed or reduced to a coal. And 

 " the quantity of mucilage, oil, or salt in fertile 

 " land, is so small, that it could not contribute 

 " the one-thousandth part of the weight of any 

 " vegetable. Whereas coal is not only supplied 

 " by the land, but also by fixed air combined 

 " with the earths, and also by that which is con- 

 " stantly let loose by various processes, and soon 

 " precipitates by superiority of its specific gra- 

 " vity, and is then condensed in or mechanically 

 " absorbed by soils, or contained in dew." 



Sir Humphry Davy says, " The necessity of 

 " water to vegetation, and the luxuriancy of the 

 " growth of plants, connected with the presence 

 " of moisture, in the southern countries of the 

 " old continent, led to the opinion so prevalent 

 " in the early schools of philosophy, that water 

 " was the great productive element, the sub- 

 " stance from which all things were capable of 

 " being composed, and into which they were 



