260 CHEMISTRY. 



pigment. It is a very poisonous substance. It is formed 

 whenever acetic acid is brought in contact with copper. 

 No article of food, then, in which there is vinegar should 

 be cooked or kept in a copper vessel. 



367. Experiment -with Sulphate of Copper. If you hold a 

 knife -blade for a few minutes in a strong solution of sul- 

 phate of copper, it will be covered with a coating of metal- 

 lic copper. 



The copper is precipitated upon the iron, while the iron 

 goes into the solution. This is shown as follows : 



Sulphate of copper. Iron. Ferrous sulphate. Copper. 

 CuSO 4 + Fe = FeSO 4 + Cu 



This experiment was tried on a large scale some years ago 

 in Ireland. In some pits at a mine in Wicklow there was 

 a large amount of the solution of sulphate of copper. In 

 order to get the metallic copper from this, 500 tons of iron 

 were placed in the pits and left there for a year. The re- 

 sult was that the iron was all united to the sulphuric acid, 

 forming ferrous sulphate, which was dissolved in the water, 

 and the metallic copper lay in the form of a reddish mud 

 at the bottom of the pits. This was taken out, and, after 

 being freed from its impurities, was melted and cast in bars. 

 The same expedient has been adopted in other mines. 



368. Test for Copper. Polished steel, as shown by the 

 experiment with the knife-blade, is a good test of the pres- 

 ence of salts of copper. If pickled cucumbers or preserved 

 fruit have been prepared in copper vessels, we can ascer- 

 tain whether copper be present in them by introducing a 

 slip of polished steel, or, what is the same thing, a bright 

 knife-blade. If there be any salt of copper, the metal it- 

 self will be deposited upon the steel. Of course, as the 

 quantity, if there be any, must be small, the steel must re- 

 main in the liquid for some little time, and the deposit must 

 necessarily be small. If the salt be acetate of copper, as it 



