OTHB CORRELATION OP THE PHYSICAL FORCES 77 



that mixture is now boiling ; but when I add a little mercury 

 to it the gas ceases to come off. We have now scarcely a 

 bubble of hydrogen set free, so that the action is suspended 

 for the time. We have not destroyed the power of chemical 

 affinity, but modified it in a wonderful and beautiful manner. 

 Here are some pieces of zinc covered with mercury exactly 

 in the same way as the zinc in that retort is covered ; and if 

 I put this plate into sulphuric acid I get no gas, but this 

 most extraordinary thing occurs, that if I introduce along 

 with the zinc another metal which is not so combustible, then 

 I reproduce all the action. I am now going to put to the 

 amalgamated zinc in this retort some portions of copper wire 

 (copper not being so combustible a metal as the zinc), and 

 observe how I get hydrogen again, as in the first instance; 

 there, the bubbles are coming over through the pneumatic 

 trough, and ascending faster and faster in the jar ; the zinc 

 now is acting by reason of its contact with the copper. 



Every step we are now taking brings us to a knowledge of 

 new phenomena. That hydrogen which you now see coming 

 off so abundantly does not come from the zinc, as it did be- 

 fore, but from the copper. Here is a jar containing a solu- 

 tion of copper. If I put a piece of this amalgamated zinc 

 into it, and leave it there, it has scarcely any action ; and here 

 is a plate of platinum which I will immerse in the same 

 solution, and might leave it there for hours, days, months, 

 or even years, and no action would take place; but, by 

 putting them both together, and allowing them to touch 

 (Fie. 44), you see what a coating of copper there is im- 

 mediately thrown down on the platinum. Why is this ? The 

 platinum has no power of itself to reduce that metal from 

 that fluid, but it has, in some mysterious way, received this 

 power by its contact with the metal zinc. Here, then, you see 

 a strange transfer of chemical force frm one metal to an- 

 other; the chemical force from the zinc is transferred and 

 made over to the platinum by the mere association of the 

 two metals. I might take, instead of the platinum, a piece 

 of copper or of silver, and it would have no action of its 

 own on this solution, but the moment the zinc was introduced 

 and touched the other metal, then the action would take 

 place, and it would become covered with copper. Now is 



