No. V.] APPENDIX. 165 



the experiment, the quantity of hydrogen evolved from a minute portion of 

 this substance, when touched with a piece of zinc, was truly astonishing. 

 The mass in this state was so fragile that the hydrogen disintegrated it 

 almost instantaneously, showing that in this form it could not be used for 

 a voltaic battery. 



My next experiments were to coat other metals with this finely-divided 

 platinum; and I found that platinum, palladium, or silver, answered 

 admirably for the reception of it, and similar help was afforded to the 

 evolution of the hydrogen, as the contrast between the gas given off from 

 the smooth metal and rough metal forms a most striking experiment. 

 Other metals received the platinum with advantage ; as plated copper or 

 iron, and even charcoal, was benefited to a similar extent.* 



The metals thus roughened by platinum have, in addition to their 

 power, some properties which are very interesting : thus, when a piece of 

 the prepared metal is placed in dilute sulphuric acid and touched with a 

 small rod of zinc, gas is not given off from its whole extent, but only from 

 the space of a sniall circle ; and when contact is completed with a smooth 

 piece of platinum, the gas will not be given off from the latter, but will 

 travel principally to the rough portion, there to be evolved. This curious 

 experiment affords a marked difference from those cases where the hydro- 

 gen is absorbed, as when a piece of silver is touched with a rod of zinc in 

 dilute sulphate of copper, for in this case an immense circle of copper will 

 be thrown down. 



A difficulty now arose in this stage of the proceeding, for the finely- 

 divided platinum was so easily rubbed off that it could not be practically 

 used with advantage. However, when the silver or other metal was first 

 roughened by the removal of the surface by an acid, then the adhesion was 

 so great that a piece of platinum thus prepared was sent accidentally to 

 the instrument-maker, where the workman mistook the finely-divided 

 platinum for dirt, and could only remove it with sand-paper. 



It now became desirable to ascertain the power of metal thus prepared 

 relatively with the other batteries, and also with metals uncovered with 

 the finely-divided platinum; and to make this comparison, I perceived that 

 considerable difficulty occurred, for as this preparation of the metals in- 

 creases the quantity, but does not interfere with the intensity, a fair com- 

 parison cannot be made where there is any impediment or difficulty to 

 be overcome, unless that difficulty be superseded by increasing the number 

 of cells of the battery : and therefore, had I at first taken the decomposi- 

 tion of water as the test for my numerous experiments, they would have 

 been attended with an immense expense ; had I taken the heating of wire 

 as my test, that would also have been uncertain, according as the heating 

 of large or small wires was estimated, but I considered that a close rela- 

 tive estimate of power could be ascertained by the magnetical effect ; for 

 by using large wires round the temporary magnet, but little impediment 

 was offered to the current, and thus the quantity, independent of the in- 

 tensity, could be accurately ascertained; and in repeating my experiments, 

 at different times, on the same magnet and with the same surface of like 



* Charcoal and plumbago might be considered to afford points enough for 

 the escape of the hydrogen, but to these there is great adhesion of the gas. 



