Producible by Attrition and Contact of Metals. 9 



I use the terms " zinc side" and " silver side." When it is mentioned that any 

 process was effected at the zinc side, it means that the substance concerned was 

 connected with that end of the galvanometer coil, which, if in communication 

 with the zinc element of a voltaic combination of zinc, silver, and dilute sul- 

 phuric acid, would cause that end of the upper bar of the compound, or astatic 

 needle, which naturally points to the north, to tui-n towards the west. The silver 

 side, therefore, means the end of the coil which, with that same deflection, 

 would be in connexion with the silver element of such a combination. 



When eastern or western deflection is mentioned, it signifies that the end of 

 the upper bar of tlie compound needle, which naturally points to the north, has 

 moved in these directions. For many of the experiments described a galvano- 

 meter of extreme susceptibility is required, in order that the results may be 

 unequivocal. I have described one in the Transactions of the Royal Irish 

 Academy, Vol. xxu., Part 3, which answers perfectly, and it was used in these 

 experiments. 



In most of the following experiments the metals to be tried were formed 

 into hemispheres, the flat faces of which were polished : they were of an inch and 

 quarter diameter across the flat circular face ; the round back of each had a neck 

 to which the rheophore wire was firmly fixed ; and each hemisphere was attached 

 by its neck to a wooden handle. The rheophore wire was generally of silvered 

 copper nearly as thin as a human hair ; for by this contrivance the portion at- 

 tached to the neck of the hemisphere always maintained the temperature of the 

 neck itself ; hence, by Law iv., the contact of the neck and rheophore did not 

 develope any deflecting agency, and there was no interference with the indica- 

 tions of the hemispheres experimented on. These extremely fine rheophore 

 wires are only fitted for metals which act on each other with a certain intensity ; 

 when the deflecting power of the metals is weak, the rheophore must consist 

 of a somewhat thicker wire ; but even such wire, when connected with the 

 hemispheres, does not interfere with the deflections. In the case of platinum, 

 palladium, and gold, I made use of thin plates of these metals fastened to 

 wooden handles. Such handles are indispensable, because, if the metallic hemi- 

 spheres, or plates, were held in the hand, their temperature would be interfered 

 with, and erroneous results obtained. 



The means made use of for bringing the hemispheres to any required tem- 



VOL. xxui. c 



