FAMILIAR LETTERS OX CHEMISTRY. 



35 



removing every obnoxious influence. No experience, drawn from the exercise of 

 the art, can be opposed to true scientific principles, because the latter should include 

 all the results of practical operations, and are in some instances solely derived there- 

 from. Theory must correspond with experience, because it is nothing more than 

 the reduction of a series of phenomena to their last causes. 



A field, in which we cultivate the same plant for several successive years, becomes 

 barren for that plant in a period varying with the nature of the soil : in one field it 

 will be in three, in another in seven, in a third in twenty, in a fourth in a hundred 

 years. One field bears wheat, and no peas ; another beans and turnips, but no 

 tobacco ; a third gives a plentiful crop of turnips, but will not bear clover. What 

 is the reason that a field loses its fertility for one plant, the same which at first 

 flourished there ? What is the reason one kind of plant succeeds in a field where 

 another fails ? 



These questions belong to science. 



What means are necessary to preserve to a field its fertility for one and the same 

 plant? what to render one field fertile for two, for three, for all plants? 



These last questions are put by Art, but they cannot be answered by Art. 



If a farmer, without the guidance of just scientific principles, is trying experi- 

 ments to render a field fertile for a plant which it otherwise will not bear, his 

 prospect of success is very small. Thousands of farmers try such experiments in 

 various directions, the result of which is a mass of practical experience forming a 

 method of cultivation which accomplishes the desired end for certain places ; but 

 the same method frequently does not succeed it indeed ceases to be applicable to 

 a second or third place in the immediate neighborhood. How large a capital, and 

 how much power, are wasted in these experiments ! Very different, and far more 

 secure, is the path indicated by SCIENCE ; it exposes us to no danger of failing, but 

 on the contrary, it furnishes us with every guarantee of success. If the cause of 

 failure of barrenness in the soil for one or two plants has been discovered, means 

 to remedy it may readily be found. 



The most exact observations prove that the method of cultivation must vary with 

 the geognostical condition of the subsoil. In basalt, graywacke, porphyry, sand- 

 stone, limestone, &c., are certain elements indispensable to the growth of plants, and 

 the presence of which renders them fertile. This fully explains the difference in 

 the necessary methods of culture for different places ; since it is obvious that the 

 essential elements of the soil must vary with the varieties of composition of the 

 rocks, from the disintegration of which they originated, 



Wheat, clover, turnips, for example, each require certain elements from the soil ; 

 they will not flourish where the appropriate elements are absent. Science teaches 

 us what elements are essential to every species of plants by an analysis of their 

 ushes. If, therefore, a soil is found wanting in any of those elements, we dis- 

 cover at once the cause of its barrenness, and its removal may now be readily 

 accomplished. 



The empiric attributes all his success to the mechanical operations of agriculture ; 

 he experiences and recognises their value, without inquiring what are the causes of 

 their utility, their mode of action : and yet this scientific knowledge is of the highest 

 importance for regulating the application of power and the expenditure of capital 

 for insuring its economical expenditure and the prevention of waste. Can it be 

 imagined that the mere passing of the ploughshare, or the harrow, through the soil 

 the mere contact of the iron can impart fertility miraculously ? Nobody, per- 

 haps, seriously entertains such an opinion. Nevertheless, the modus operandi of 

 these mechanical operations is by no means generally understood. The fact is 

 quite certain, that careful ploughing exerts the most favorable influence : the sur- 

 face is thus mechanically divided, changed, increased, and renovated : but the 

 ploughing is only auxiliary to the end sought. 



In the effects of time, in what in Agriculture are technically called fallows the 

 repose of the fields we recognise by science certain chemical actions, which are 

 continually exercised by the elements of the atmosphere upon the whole surface of 

 our globe. By the action of its oxygen and its corbonic acid, aided by water, rain, 

 changes of temperature, &c., certain elementary constituents of rocks, or of their 

 ruins, which form the soil capable of cultivation, are rendered soluble in water, and 

 consequently become separable from all their insoluble parts. 



These chemical actions, poetically denominated the " tooth of time," destroy all 

 the works of man, and gradually reduce the hardest rocks to the condition of dust. 

 By their influence the necessary elements of the soil become fitted for assimilation 

 by plants ; and it is precisely the end which is obtained by the mechanical opera- 

 tions of farming. They accelerate the decomposition of the soil, in order to provide 

 a new generation of plants with the necessary elements in a condition favorable to 



