342 SCIENCE OF AGRICULTURE. Part II. 



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are nitre, or the nitrous acid combined with potassa. Sir Kenelm Digby states, that he 

 made barley grow very luxuriantly by watering it with a very weak solution of nitre ; but 

 he is too speculative a writer to awaken confidence in his results. This substance con- 

 sists of one proportion of azote, six of oxygen, and one of potassium ; and it is not 

 unlikely that it may furnish azote to foriji albumen or gluten in those plants that contain 

 them ; but the nitrous salts are too valuable for other purposes to be used as manures. 

 Dr. Home states, that sulphate of potassa, which was just now mentioned as found in 

 the ashes of some peats, is a useful manure. But Naismith {^Elements of Agriculture^ 

 p. 78.) questions his results ; and quotes experiments hostile to his opinions, and, as he 

 conceives, unfavorable to the efficacy of any species of saline manure. Much of the 

 discordance of the evidence relating to the efficacy of saline substances depends upon 

 the circumstance of their having been used in different proportions, and, in general, in 

 quantities much too large. 



2241. Solutions of saline substances were used twice a week, in the quantity of two 

 ounces, on spots of grass and corn, sufficiently remote from each other to prevent any in- 

 terference of results. The substances tried were super-carbonate, sulphate, acetate, nitrate, 

 and muriate of potassa ; sulphate of soda ; sulphate, nitrate, muriate, and carbonate of am- 

 monia. It was found, that in all cases when the quantity of the salt equalled one thirtieth 

 part of the weight of the water, the effects were injurious ; but least so in the instance of 

 the carbonate, sulphate, and muriate of ammonia. When the quantities of the salts were 

 one three-hundredth part of the solution, the effects were different. The plants watered 

 with the solutions of the sulphates grew just in the same manner as similar plants watered 

 with rain-water. Those acted on by the solution of nitre, acetate, and super-carbonate 

 of potassa, and muriate of ammonia, grew rather better. Those treated with the solution 

 of carbonate of ammonia grew most luxuriantly of all. This last result is what might be 

 expected, for carbonate of ammonia consists of carbon, hydrogen, azote, and oxygen. 

 There was, however, another result which was not anticipated ; the plants watered with 

 solution of nitrate of ammonia did not grow better than those watered with rain-water. 

 The solution reddened litmus paper ; and probably the free acid exerted a prejudicial 

 effect, and interfered with the result. 



2242. Soot doubtless owes part of its efficacy to the ammoniacal salts it contains. The 

 liquor produced by the distillation of coal contains carbonate and acetate of amonia, and 

 is said to be a very good manure. 



2243. Soapers* waste has been recommended as a manure, and it has been supposed 

 that its efficacy depended upon the different saline matters it contains ; but their quantity 

 is very minute indeed, and its principal ingredients are mild lime and quick-lime. In 

 the soapers' waste, from the best manufactories, there is scarcely a trace of alkali. Lime, 

 moistened with sea-water, affords more of this substance, and is said to have been used 

 in some cases with more benefit than common lime. 



2244. The result of Sir H. Davy's discussion as to the extent of the effects of saline sub- 

 stances on vegetation^ is, that except the ammoniacal compounds, or the compounds con- 

 taining nitric, acetic, and carbonic acid, none of them can afford by their decomposition 

 any of the common principles of vegetation carbon, hydrogen, and oxygen. The 

 alkaline sulphates and the earthy muriates' are so seldom found in plants, or are found in 

 such minute quantities, that it can never be an object to apply them to the soil. The 

 earthy and alkaline substances seem never to be formed in vegetation ; and there is every 

 reason to believe, that they are never decomposed ; for, after being absorbed, they are 

 found in their ashes. The metallic bases of them cannot exist in contact with aqueous 

 fluids ; and these metallic bases, like other metals, have not as yet been resolved into any 

 other forms of matter by artificial processes ; they combined readily with other elements ; 

 but they remain indestructible, and can be traced undiminished in quantity, through their 

 diversified combinations. 



Chap. III. 



Of the Agency of Heat, Light, Electricitj/y and Water, in Vegetable Culture. 



2245. The particular agency of heat, light, and water in vegetation and culture has 

 been so frequently illustrated, that it only remains to give a general idea of their natures, 

 and to offer some remarks on electricity. 



Sect. I. Of Heat and Light. 



2246. Tlie heat of the sun is the cause of growth, and its light the cause of maturity, in the 

 vegetable kingdom. This is universally acknowledged : animals will live without or with 

 very little light ; but no plants whatever can exist for any time without the presence of 

 this element. The agency of electricity in vegetation is less known. 



