96 COATING OF IRON WITH COPPER. 



the iron, that it should have a considerable thick coating; and, as 

 the cyanide process is expensive, it is preferable, when the iron has 

 received a film of copper by the cyanide solution, to take it out, 

 wash it in water, and attach it to a single cell or weak battery, and 

 put it into a solution of sulphate of copper. If there is any part not 

 sufficiently covered with copper by the cyanide solution, the sul- 

 phate will make these parts of a dark colour, which a touch of the 

 finger will remove. When such is the case, the article must be 

 taken out, scoured, and put again into the cyanide solution till per- 

 fectly covered. A little practice will render this very easy. The 

 sulphate solution for covering iron should be prepared by adding to 

 it by degrees a little caustic potash, so long as the precipitate 

 formed is re-dissolved. This neutralizes a great portion of the sul- 

 phuric acid, and thus the iron is not so readily acted upon. 



Effects of Conducting Power in Solutions and Metals. In cover- 

 ing iron, platinum, or such comparatively bad-conducting metals, 

 with other metals that are good conductors, or the solutions of 

 which are good conductors, the property of conduction in relation 

 to the solution is beautifully illustrated. If we take a copper wire, 

 say 8 or 10 feet long, one end of which is attached to the zinc of a 

 battery, and laid parallel with the positive electrode into a solution 

 for the purpose of receiving a deposit, it will be found that the 

 greatest amount of deposit has taken place at the end furthest from 

 the battery: but if an iron or platinum wire be substituted for the 

 copper one, the contrary result will take place ; for the end furthest 

 from the battery will be the last to receive the coating, and will 

 have the least quantity of metal deposited upon it. If the copper 

 wire were 30 feet long, little alteration would be seen in the deposit; 

 but upon an iron or platinum wire of that length the deposit pro- 

 ceeds only a certain distance, and no deposit will take place on the 

 end furthest from the battery until the current has passed a con- 

 siderable time, after which the deposit is observed to advance gradu- 

 ally. The copper as it becomes deposited on the iron acts as a 

 conductor, transmitting the deposit further onwards to its final 

 point, as well as adding to the deposition already effected upon the 

 iron. The length of deposit that would be formed on the first 

 immersion of the wire depends upon the conducting power of the 

 solution ; for, as already stated, solutions vary in this property as 

 well as metals. We have found that a few feet of iron wire offer a 

 greater resistance to the passage of the current than the solution 

 between the iron wire and the positive electrode, which is only 

 about 2 or 3 inches ; but their exact relations to each other we 

 have not yet had an opportunity of investigating. 



Under these circumstances, it may be asked, why no.t increase 

 the intensity of the battery, and so force it along the wire? But 



