Profs. W. E. Ayrton and John Peny. [Mar. 21, 



zinc vessel for many hours. In fact in chemical analysis, something 

 which is equivalent to current is measured by a method very much less 

 sensitive than if the measurement were made by the best galvanometer, 

 whereas a current which is not observable with the best galvanometer 

 may be measured accurately with the electrometer. 



It is plain that if we assume an electromotive force of contact be- 

 tween any two substances, there must be possible chemical action 

 between these two substances, and this chemical action can of course 

 only exist at the places of contact. It may proceed slowly through 

 want of conductivity, and the amount possible in any given case may 

 be very small because of want of mobility of either of the substances 

 in contact, or of want of mobility of the compound formed by the 

 action. Thus it is well known that the electromotive force of contact 

 of glass and other substances is often very considerable, and that the 

 chemical action between glass and most other substances is very small. 

 This is due to want of conductivity of the glass, but when glass 

 becomes heated, its conductivity much increases, and it then becomes 

 capable of acting chemically on many substances. When in the 

 melted state it is acted on chemically by the majority of substances. 

 It is possible that this great increase of chemical activity accompany- 

 ing a rise of temperature is much more due to increased conductivity 

 than to change of electromotive force of contact. Lead glass which 

 insulates so badly, is comparatively easily acted on by chemicals. 

 Chemical action is always slow unless the substances are conductors, 

 and we find that nearly all the substances which chemists find it un- 

 necessary to dissolve before applying re-agents are conductors. In 

 some cases previous solution may be also required for the mere pur- 

 pose of producing mobility at the surfaces of contact. To expedite 

 chemical action, solutions are stirred together ; this increases the area 

 of contact and diminishes resistance as well as rapidly bringing into 

 contact fresh portions previously unacted on. 



The alkaline earths in the solid state are very bad conductors, but 

 when melted they are comparatively good conductors, and there is no 

 doubt that we have here an explanation of the fact that the bottoms 

 come out of platinum crucibles containing melted alkaline earths. 

 We might expect that in those cases in which melted metals have a 

 considerable electromotive force of contact, they would combine to 

 form alloys with great rapidity, on account of their great conductivity 

 and sufficient mobility to continue chemical combination. The great 

 conductivity of mercury, and its mobility and the mobility of the 

 compounds which it forms with other metals, enables it to enter 

 rapidly into combination with those metals with which it has a great 

 electromotive force of contact. And it is probable that the electro- 

 motive force of contact of mercury with iron or with steel is exceed- 

 ingly small, or that the amalgam is a solid except at very high tern- 



