Chemistry and Physics. 31£ 



4. On the divergence of Electromotive forces from Thermo- 

 Chemical data. — Professor E. F. Herroun, sums up his research 

 as follows : — 



(1.) The primary factor in determining the electromotive force 

 of a Voltaic cell is the relative heat of formation of the anhy- 

 drous salts of the two metals employed. 



(2.) That the E. M. F. may set up chemical changes of a differ- 

 ent direction and character from those predicable from the heat 

 of formation of the dissolved salts. 



(3.) That the E. M. F. set up by (1) may be, and usually 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 dissolved salts. 



(4.) That in those cases in which there is no chemical attrac- 

 tion, 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 forward. 



(5.) That when metals, whose salts have purely negative heats 

 of solution, are oppposed 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 forward. 



(6.) 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 

 electrical energy working at uniform temperature (negatives the 

 supposition concerning mercury and other salts.) 



(7.) 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 diffei'ent thermo chemical 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, mercury in mercuric chloride, probably silver 

 in most soluble chlorides). 



(8.) That the electromotive force of a voltaic cell furnishes a 

 more accurate measurement of this free energy, and therefore of 

 true chemical affinity, than data derived from calorimetric obser- 

 vations." — Phil. Mag., March, 1889, pp. 209-233. J. t. 



5. Behavior of Metals to Light. — At a meeting of the Physi- 

 cal Society, held in Berlin, Jan. 11, Kundt gave an account 

 of his experiments on the refraction of light by metals. Metals 

 whose refractive index is large, showed an increase of the angle 

 of deviation of light as the temperature rises; thus proving 

 that the author was dealing with true refraction. A further out- 

 come of his experiments was to show that the velocity of light in 

 metals is dependent on changes of temperature in a way exactly 

 similar to that in which their electrical conductivity is depend- 

 ent. — Nature, Feb. 7, 1889, p. 360. j. t. 



