70 Prof. W. Ostwald on the Seat of the 



point, the rotations of the molecules, seems to be especially 

 suited to this purpose. 



§9. 



The behaviour of bodies under changes of temperature is 

 quite different from their magnetic and mechanical behaviour. 



If a body, whether deformed or magnetized, has been ac- 

 commodated by repeated heating and cooling, so that the 

 molecules have assumed their final mean position of equili- 

 brium for each single temperature degree, the same mechanical 

 or magnetic condition corresponds to the same temperature, 

 whether with rising or falling temperature. Thus a thermo- 

 meter, after repeated changes of temperature, gives the same 

 indications at the same temperature, whether with rising or 

 falling temperature, when once the extreme indications have 

 become constant. So also a thermo-element (say of copper 

 and german silver) shows, after accommodation, always the 

 same electromotive force, and a magnet always the same per- 

 manent moment at a given temperature, as is shown by direct 

 experiment. The difference is just this, that in mechanical 

 deformation and magnetization the molecules have been 

 carried over into new more or less stable positions of mecha- 

 nical equilibrium, by means of mechanical displacements and 

 rotations, out of which they may be directly displaced by 

 fresh mechanical influences ; whereas by heating we change 

 only the amplitude of the vibrations of the molecules in all 

 directions above the same position of equilibrium. 



VIII. On the Seat of the Electromotive Forces in the Voltaic 

 Cell By Prof. W. Ostwald. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



IN accordance with a request made in the annexed portion 

 of a letter just received, may I ask you to insert it in 

 your next issue. 



Your obedient servant, 



Olivek Lodge. 



Riga, June 2, 1886. 

 Sir, — After reading your memoir on the seat of E.M.F. in 

 the pile, published in last year's Philosophical Magazine*, allow 

 me to express my full agreement with your views. But I 

 further believe that it will interest you to know that there is a 

 method of directly measuring differences of potential, whether 

 between two liquids or between a liquid and a metal. 

 * Phil. Mag. October 1885, p. 372. 



