86 



ELECTIVE AFFINITY. 



nation of two powerful poisons {chlorine and so* 

 dium) . 



When different substances are brought together, 

 the results will vary according to the peculiar prop- 

 erties of each. Decompositions of one or more 

 will take place, and new compounds be formed, and 

 on this fact is founded the doctrine of elective affin- 

 ity. To illustrate : Many years ago, some workmen 

 employed in the copper-mines of Wicklow left their 

 iron shovels in a place where they were exposed to 

 running water containing copper in solution. The 

 result was, that the shovels, when found some time 

 after, were covered with copper to such an extent, 

 that the workmen were induced to believe that they 

 had been wholly changed into copper. In conse- 

 quence of this fact being observed, about 500 tons 

 of iron were introduced into the mines, and at the 

 end of a year the iron was found dissolved ; every 

 ton of iron produced a ton, sometimes a ton and 

 a half, and even two tons, of metallic precipitate ; 

 and from each ton of this substance were produced 

 16 cwt. of pure copper.* This circumstance was 

 owing to the fact that the copper was dissolved in 

 sulphuric acid ; and as this acid has a stronger affin- 

 ity for iron than for copper, it united with the for- 

 mer, leaving the copper adherent to its surface. This 

 experiment is easily performed by dissolving a bit 

 of copper in sulphuric acid, and immersing in it a 

 piece of iron. Compound elective affinity is where 

 a double decomposition takes place ; as, for ex- 

 ample, if we mix together sulphate of potassa and 

 nitrate of soda, there result sulphate of soda and 

 nitrate of potassa, the sulphuric acid leaving the 

 potash and uniting with the soda, and the nitric acid 

 leaving the soda and uniting with the potash. This 

 is a complex operation, but one which we witness 

 every day. If we turn a solution of camphor into 



* Von Leonhard's Lectures, 



