338 Philosophy of Manures. 



health. Manures, therefore, abounding in nitrogen exclusively, 

 as nitrates of soda and potash, should increase the plant more 

 by stimulating the tissue to expand in bulk, and abound more in 

 soft mucilage than ripened starch; unless where there is a due 

 proportion of carbon already in the soil, and where light and 

 heat are found in sufficient quantity to elaborate the food of the 

 plant. 



A stimulating food will stimulate the action of the organs, 

 and may cause greater results from the same quantity of 

 food. The animal increases in bulk, not according to the 

 quantity of food swallowed, but according to the activity of the 

 digestive and absorbent system (the stomach and lacteals), and 

 assimilating organs. In like manner, the greater activity of the 

 absorbent and assimilating system of plants should produce 

 more food, if the carbon and other constituents are present; 

 but it should be on land otherwise rich, and mixed with 

 other manures, that ammonia or nitrogen should produce its 

 full effect. The action of the nitrates has been stated by some 

 as being stimulating on the matters in the soil, or, rather, they 

 seem to mean helping to reduce the vegetable matter in the soil, 

 as lime ; but this the nitrates cannot do till separated from the 

 nitric acid, which neutralises them, perhaps by being decomposed 

 in the plant, and again excreted. Their principal benefit, in the 

 first place, is likely to arise from furnishing nitrogen, a stimulat- 

 ing food, to the plant ; and they should, as before observed, be 

 mixed with other manures, or applied to otherwise rich land, to 

 produce their full and proper effect. Mr. Locke, gardener to 

 Archibald Hamilton, Esq., of Roselle, informs me that these 

 manures have been largely experimented on, in various ways, 

 both on the farm and in the garden at Roselle this year ; and 

 to the public spirit of that gentleman we are likely to be 

 indebted for some interesting information on this head. 



Professor Daubeny seems still doubtful as to the truth of the 

 theory of the Excretions of Plants by the Roots ; and Professor 

 Lindley lately, in the Chronicle, seems to participate in the same 

 opinion. I mentioned in my former essay that I considered 

 the decreasing quantity of alkali in wheat, as it ripened, a proof 

 of the excretory theory being correct. If we allow that the 

 roots imbibe every thing soluble in water, which is now gene- 

 rally conceded*, we must admit that much will be taken up which 

 cannot be assimilated in the plant ; much, also, of what is capa- 

 ble of being assimilated, when in certain proportions, will often 



* Carbonic acid will pass through some membranes, as bladders, that will 

 not allow hydrogen to pass ; and spirits of wine and water are similarly 

 situated. A discriminating power has been inferred in the tissue of the 

 spongiole from this ; but all substances fairly dissolved, not suspended only, 

 in water, should pass along with the water. 



