384 Intelligence and Miscellaneous Articles. 



experiment was rather indefinite ; it scarcely did more than 

 tell us — here is the possibility of a chemical process, here 

 electricity can be produced. But which kind, how much, to 

 what potential, remained indefinite. I have not found in all 

 the papers which have been written for the defence of the 

 chemical theory a clear explanation why zinc opposed to cop- 

 per in liquids, where zinc really is oxidized and dissolved, 

 becomes negative, and why in air and other gases it becomes 

 positive, if the same cause (viz. oxidation) is at work." 



To the chemical strain theory, held by me, none of this 

 remark is in the least applicable. This theory does not say 

 that zinc opposed to copper in a liquid is negative. It says 

 that, changing the surrounding medium from air to acid-water, 

 makes scarcely any difference : and that accordingly connect- 

 ing zinc and copper by a drop of water leaves them with the 

 same respective potentials as they had before. The strain of 

 oxygen-atoms towards a metal exists whether in air or water, 

 but it can produce no effect in either case until at some point 

 the oxygen be swept away by contact with another metal. 

 Then instantly the chemical forces are able to do work, and to 

 produce, if in air or other dielectric, the Volta-effect ; if in acid 

 or other electrolyte, the voltaic current. The metallic con- 

 tact equalizes the potential of the two metals in the one case; 

 in the other it perpetually fails to equalize their potential. 

 The equalization of an otherwise disturbed potential is its only 

 effect. 



And no indefiniteness exists as to either the kind or amount 

 of the Volta-effect as so produced, for I have shown fully that 

 a Volta-effect can be calculated in absolute measure for any 

 pair of clean metals immersed in any medium, from purely 

 thermo-chemical data ; but whether this effect is the real one 

 or not is at present a matter of opinion. 



These so-calculated effects undoubtedly agree in some cases 

 with experimentally observed ones ; but whether they all so 

 agree, and, if not, the extent to which they are erroneous, are 

 matters mainly for future experiment to decide; and upon 

 such agreement or disagreement between calculation and 

 experiment my theory definitely stands or falls. 



XLVI. Intelligence and Miscellaneous Articles. 



ON A DIFFERENTIAL RESISTANCE THERMOMETER. 

 BY T. C. MENDENHALL. 



r PHE determination or registration of the temperature at a distant 

 '-*■ or not easily accessible point is so extremely desirable that 

 many methods for accomplishing this end have been proposed, and 



