Book III. HEAT AND LIGHT. 319 



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

 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, 

 unfavourable 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. 



2S08. 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 

 interference of results. The substances tried were bi-carbonate, sulphate, acetate, nitrate, 

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

 of ammonia. 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 carbonate 

 of potass, 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. 



2309. 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 ammonia, and 

 is said to be a very good manure. 



2310. 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 quicklime. 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. 



23 11. The result of Sir H. Davy^s discussion as to the extent of the ejects 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, viz. carbon, hydrogen, and oxygen. TTie 

 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 yegetation ; and there is every 

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

 found in the 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 combine readily with other elements, 

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

 diversified combinations. 



Chap. II L 



Of the Agency of Heat, Lightf Electricity, and WcUer, in Vegetable Culture. 



2312. 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 natiu'es, 

 and to offer some remarks on electricity. 



Sect. I. Of Heat and Light. 



2313. The 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 

 light or -with very little ; but no plants whatever can exist for any time without the pre- 

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



2314. Two opinions are current respecting the nature of heat. By some philosophers it 

 is conceived to be a peculiar subtile fluid, of which the particles repel each other, but 

 have a strong attraction for the particles of other matter : by others it is considered as a 

 motion or vibration of the particles of matter, which is supposed to differ in velocity in 



