160 SMITH'S INTERMEDIATE CHEMISTRY 



arranged in the form of a battery-cell. The order of activity of the 

 metals, given on p. 54, is obtained by this third method. 



It is evident that the chemical activity or affinity of a given 

 substance will not be the same towards all others. Thus, iron 

 unites much more vigorously with chlorine than with sulphur and, 

 with identical amounts of iron, more heat is liberated in the former 

 case than in the latter. With silver, sodium, and many other 

 substances, iron does not unite at all. One of the tasks of the 

 chemist is to make such comparisons as this. He calls the results, 

 the specific chemical properties of the substances in question. 



Care must be observed, however, in making comparisons by 

 the above methods. Although it is true that most chemical 

 changes that take place readily develop heat, yet at high tem- 

 peratures compounds can be formed by the direct union of their 

 elements with absorption of heat (see, for example, p. 312). Re- 

 versible reactions must also be accounted for. In such reactions, 

 one of the chemical changes taking place necessarily absorbs 

 just as much heat as the other develops, yet both have a definite 

 speed under any given conditions, as is shown by the fact that 

 a fixed point of equilibrium is reached. Deacon's process (p. 140) 

 is a good instance. As we shall see later (chapter XX), the relative 

 concentrations of the reacting substances, as well as their affinities, 

 must be taken into consideration. 



The Cause of Chemical Activity or Affinity. The 



reader will undoubtedly be inclined to inquire whether we can 

 assign any cause for the tendency which substances have to under- 

 go chemical change. Why do iron and sulphur unite to form 

 ferrous sulphide, while other pairs of elements taken at random 

 will frequently be found to have no effect upon one another under 

 any circumstances? A final answer to this question cannot, of 

 course, be given. As the facts regarding chemical activity or 

 affinity, however, become better known, we may arrive at a 

 stage where a logical explanation (see p. 23) can be advanced which, 



