386 Messrs. Wright and Thompson on the Determination of 



Analogous results are obtained when the calculations are 

 similarly made for other kinds of mixture : the observed 

 E.M.F. generally notably exceeds that calculated from the 

 heat of intermixture*. It would hence seem that the modus 

 operandi of a "diffusion-cell" is to some extent rather analo- 

 gous to that of a thermo-couple than to that of an ordinary 

 galvanic element, part at least of the energy gained outside 

 the battery being due, not to the heat-development during 

 chemical action taking place, but to the transformation of 

 sensible heat into electric-current energy. It is noteworthy, 

 moreover, that analogous phenomena are presented in cases 

 where strong and weak solutions of acids (e. g. nitric and sul- 

 phuric) interdiffuse, platinum or other unoxidizable plates 

 being employed : diffusion-cells of this kind are now being 

 examined. 



By uniting together in series a sufficient number of diffusion- 

 cells, all the effects of an ordinary voltaic battery can be pro- 

 duced, at least those compatible with the presence of a large 

 resistance in circuit : electrolysis of metallic solutions (e,. g. 

 copper sulphate) is readily brought about. 



Volume-alteration on Intermixture of Solutions of Zinc, Copper, 

 and Cadmium Sulpjhates. 



162. Although not directly connected with the subject of 

 electrical measurement of chemical affinity, it yet seemed of 

 some interest to examine how far the volume-alterations that 

 take place on mixing solutions of different strengths of zinc 

 (copper or cadmium) sulphate run parallel with the thermal 

 developments thereby caused. It might reasonably be anti- 

 cipated that the much greater heat-evolution taking place on 

 dilution of cadmium sulphate as compared with zinc or copper 

 sulphate would be found to be concurrent with a greater 

 amount of volume-alteration. On examining the matter, how- 

 ever, this is not found to be the case ; the volume-alterations 

 on dilution through a given range being but little different, 

 whichever of the three sulphates be employed, in every case 



* Since these experiments were made, the third volume of Julius 

 Thomsen's Thermochetmsche Unter&uchungen has appeared; in this are 

 giveu tire results of this author's previously unpublished experiments on 

 the heat of dilution of .solutions of zinc and copper sulphates, and of zinc 

 chlorides. These figures do not materially diner from those cited above 5 

 and their use, instead of those employed id this paper, makes no note- 

 worthy difference in the end conclusion that the heat of intermixture 

 usually represents a less E.MJ?. than that actually set up in any given 



