INTRODUCTION. 21 



rapidly effected. In some cases two compounds are mutually 

 decomposed, and two new compounds formed ; hence, as the 

 force of these affinities is well known, we may calculate their 

 influence in the soil, and to some extent in the organs of 

 plants. We have only to ascertain of what the soil is compo- 

 sed, to know what changes are going forward in it, and what 

 decompositions and recompositions will take place, when 

 we add earths, salts and manures to it ; and hence we are 

 furnished with the means of producing any effect that we 

 wish, and of securing the action of the proper agents upon 

 the expected crop. 



But to show further the agency of affinity, it will be neces- 

 sary to notice some of the laws which govern it, and the other 

 agents which modify its action. 



When bodies combine by the force of affinity, they do 

 not generally unite in any and every quantity or proportion, 

 but are governed by strict laws, that is, definite quantities of 

 each are required to complete their union ; thus water is 

 formed by the union of 1 part by weight of hydrogen and 

 8 parts of oxygen ; so carbonic acid (or Jized air) is composed 

 of exactly 6.12 parts by weight of carbon and 16 parts of oxy- 

 gen, and if the proportions of these substances are changed, 

 some other substances will be formed, but neither water nor 

 carbonic acid. Similar laws are observed when bodies com- 

 bine by volume or measure, and these laws extend to the 

 greater number of compounds both organic and inorganic ; 

 hence, as these quantities are all determined (the smallest in 

 which any body combines being called its equivalent or pro- 

 portional), we may not only explain changes in the process 

 of vegetation, and deduce important laws which tend to satis- 

 fy the mind in its investigations, but, in a more practical way, 

 we can determine the quantities of different substances which 

 any particular soil may require, especially when one saline 

 compound is substituted for another. Thus, for example, 

 when salts of ammonia are applied to the soil for the purpose 

 2* 



