Book II. HEAT AND LIGHT. 249 



part of the weight of the water, the effects were injurious ; but least so in the instances 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 sol ution reddened litmus paper ; and probably the free acid exerted a prejudicial effect, 

 and interfered with the result. 



1 1 92. 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. 



1 193. Soapers' ivaste 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. 



1 1 94. The result of Sir H. Davys 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 mi- 

 nute 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 combine readily with other elements ; but they re- 

 main indestructible, and can be traced undiminished in quantity, through their diversi- 

 fied combinations. 



Chap. III. 



Of the Agency of Heat, Light, Electricity, and Water, in Vegetable Culture. 



1 1 95. 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. 



1196. 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 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. 



1197. 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 mo- 

 tion or vibration of the particles of matter, which is supposed to differ in velocity in dif- 

 ferent cases, and thus to produce the different degrees of temperature. Whatever deci- 

 sion be ultimately made respecting these opinions, it is certain that there is matter moving 

 in the space between us and the heavenly bodies capable of communicating heat ; the mo- 

 tions of which are rectilineal : thus the solar rays produce heat in acting on the surface of 

 the earth. The beautiful experiments of Dr. Herschel have shown that there are rays 

 transmitted from the sun which do not illuminate, and which yet produce more heat than 

 the visible rays ; and Ritter and Dr. Wollaston have shown that there are other invisible 

 rays distinguished by their chemical effects. 



1198. Heat is radiated by the sun to the earth, and if suffered to accumulate, Dr. Wells 

 observes, would quickly destroy the present constitution of our globe. This evil is pre- 

 vented by the radiation of heat from the earth to the heavens, during the night, when it re- 

 ceives from them little or no heat in return. But, through the wise economy of means, 

 which is witnessed in all the operations of nature, the prevention of this evil is made the 

 source of great positive good. For the surface of the earth, having thus become colder 



