ELECTIONS AND PRECIPITATIONS. 251 



It therefore follows, that if the caloric of nitric 

 acid enabled it to dissolve and combine with the 

 silver, it must have a still stronger attraction for 

 the particles of mercury, or it would not desert 

 the atoms of silver for them ; in short, that the 

 silver is separated from its combination with the 

 acid by an abstraction of caloric, for the same 

 reason that salts dissolved in water are precipi- 

 tated by a reduction of temperature. 



The same principle must apply to all other 

 elections and precipitations, modified, however, 

 by circumstances not yet fully understood. 



Lead precipitates mercury, and copper lead, 

 which is again thrown down from a state of com- 

 bination with nitric acid by iron ; all of which 

 changes are attended with transitions of caloric 

 from the solvent to the solvend. 



The art of coating metals with other metals 

 by precipitation, is founded on the above law, 

 such as gilding, silvering, tinning, &c. If a 

 plate of copper or iron be immersed in a solution 

 of nitrate of mercury, a coating of the latter 

 metal is precipitated on them, for the same 

 reason that aqueous vapour is precipitated on 

 cold bodies, and coheres to them in the form of 

 frost or ice. If nitrate of silver be dissolved in 

 water, and a plate of clean copper be inserted 

 into the solution, a beautiful coating of silver is 

 precipitated on the copper ; or, if a polished iron 

 rod be immersed in a solution of nitro-muriate 



