166 APPENDIX. [No. V. 



metals, I found that they coincided with remarkable accuracy, and only 

 one cell was required for the experiment. Though the weight, which was 

 supported even by a small magnet with large wires, was inconveniently 

 great, I determined to ascertain the distance at which a small but lesser 

 weight was attracted. 



The following are the results of like surface of metal with the same 

 metal : 



Layers of paper. 



Smooth silver, supported keeper through ... 2 



Smooth copper 1 



Silver heated, quenched in acid 9 



surface removed by nitric acid .... 9 



Iron rough 8 



Daniell's battery 6 



Platinized silver 20 



iron, two or three varieties .... 20 



platinum '.18 



Grove's battery 26 



platinized platinum 30 



Plain platinum heated, quenched in acid ... 12 



By these experiments we see the great advantage of the rough metals 

 and those covered with platinum over the smooth metals and Daniell's 

 arrangement. 



The only metal which may take the place of finely-divided platinum is 

 palladium, but probably rhodium, iridium, and osmium would have the 

 same property, as they are precipitated in a fine black powder by zinc. 

 The cause of this black colour is not at all evident ; and the form of the 

 black deposit has eluded not only my own but the observation of others, 

 although aided by the microscope. Probably, however, the colour is owing 

 to the particles being too small to reflect the light, as is said to be the case 

 with a specimen of quartz in the cabinet of the Duchess of Gordon, but 

 this is merely hypothetical. 



We have now seen that platinum, palladium, silver, plated copper, or 

 iron, are suitable for the finely-divided metal, and these are to be first 

 roughened, the two former with sand-paper, and the three latter with a 

 little nitric acid, which is to be again cleaned off by washing. The metals 

 are then to be placed in any convenient vessel with a little dilute sulphuric 

 acid, to which a small quantity of nitro-muriate of platinum has been 

 added ; a porous tube or paper bag, containing a piece of zinc, with more 

 dilute sulphuric acid, is also to be placed in the vessel, when, as soon 

 as the circuit is completed, the platinum is precipitated on the metal 

 placed for its reception. The cost of this process will be best under- 

 stood by mentioning that the assayers sell one ounce of the prepared 

 silver for one shilling above the price which is charged for the silver 

 alone. 



The zinc which is used for the battery should be the best thick rolled 

 zinc, as this is far preferable to the cast zinc, and it is to be amalgamated 

 with mercury aided by dilute sulphuric acid ; for the application of this 

 process to the zinc of my battery will be found, unlike other batteries, not 

 to require repeating. 



