36 
ELECTIVE AFFINITY. 
nation of two powerful poisons (chlorine and so^ 
dium), M 
When different substances ar^ brought together, 
the results will v!ary 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 w^ere 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. 
