Efects of Contraction during tJie Cooling of Granite. 233 



is, supplemented by the energy, or a portion of the energy, due 

 to the hydration or solution of the solid salts, and may have 

 values which accord with the heat of formation of the dis- 

 solved salts. 



IV. That in those cases in which there is no chemical 

 attraction, or a very feeble attraction between the water and 

 the salt, the negative heat of solution is derived from sensible 

 heat, and is not supplied by the free energy of the chemical 

 change. All cells in which such salts are employed opposed 

 to zinc should have negative " thermo voltaic constants/'' and 

 evolve heat when they send a current forwards. 



V. That when metals, whose salts have purely negative 

 heats of solution, are opposed to metals whose salts they can 

 replace, the E.M.F. set up is in excess of the total thermal 

 change. Such cells, therefore, absorb sensible heat when 

 worked forwards. 



VI. That, taking the foregoing facts into consideration, no 

 cell exists which can furnish an E.M.F. in excess of the free 

 energy of the chemical change ; i. e. which can convert 

 sensible heat into electric energy working at uniform tem- 

 perature. (Negatives the supposition concerning mercury 

 and other cells.) 



VII. That certain metals have a tendency to form films of 

 sub-salts on their surfaces, the formation of which giving 

 rise, as it does, to a different thermochemical reaction, 

 naturally furnishes an E.M.F. which does not correspond 

 with the values calculated from the heats of formation of 

 their normal salts. {Ex.gr. copper in cupric chloride, mer- 

 cury in mercuric chloride, probably silver in most soluble 

 chlorides.) 



VIII. That the electromotive force of a voltaic cell fur- 

 nishes a more accurate measurement of the " free energy ,^^ 

 and therefore of true chemical affinity, than data derived 

 from calorimetric observations. 



XXVI. A Consideration of the Effects of Contraction during 

 the Cooling of Intrusive Masses of Granite and the Cause of 

 their Solid Continuity. By T. Mellakd Reade, C.E., 

 F.G.S.,F.E.LB.A.* 



"VTO doubt many geological observers have wondered, on 

 -1-^ looking at an intrusive granite- mass such as Cairnsmore 

 of Fleet in Kirkcudbrightshire, or that of Shap Fells in 



* Communicated by the Author. 



