372 Prof. 0. Lod&e on the Cont rover 



& 



there is nothing out of the way in a molecular estimate of 

 10~ 9 . On the other hand, an estimate of 10~ 8 yields a rise of 

 temperature of 79°, which appears to agree very well with 

 the order of magnitude (36° Roberts-Austen, 77° Dr. Gait) 

 obtained in actual experiments on the heat of formation of a 

 zinc-copper alloy, a truly remarkable coincidence. One, 

 however, that would be spoilt if the molecular-dimensions' 

 estimate were only halved, for the theoretical rise would then 

 be over 300°. 



With deference I submit that the air-films, with which the 

 metals employed in ordinary voltaic experiments are cer- 

 tainly coated, ought to be left on them for the data to be 

 applicable ; and in that case, when the molecular approach is 

 made, the metals are not so much alloyed as burnt ; so that 

 a rise of temperature of several hundred degrees is reasonable, 

 and a corresponding molecular estimate of say half 10~ 8 is also 

 reasonable and likely. But in the metallurgical making of 

 brass it is not to be supposed that the metals entering into 

 combination are coated with air-films of any importance, for 

 their proportion of free surface is insignificantly small. 

 Hence I think the Volta efTect has nothing really to do with 

 the making of brass. 



To carry out the above combustion process, that is the voltaic 

 approach of metallic foil of molecular thickness and its 

 investing oxygen coat, it is not necessary to complicate matters 

 by using two metals ; and in the Phil. Mag. for May 1885, 

 p. 364, I endeavoured to make a somewhat precise estimate 

 of molecular magnitudes in this way, and even to discriminate 

 between the molecular dimensions of different metals, since x 

 comes out proportional to the square root of the atomic weight 

 divided by the square root of the density and multiplied by 

 the square root of the heat of oxidation. 



Chapter III. 

 Statement of the present condition of the Controversy. 



The opposing sides of the old controversy used to be called 

 contact theorists and chemical theorists. If this were ever 

 a correct designation it is not the correct designation now. 



There is an E.M.F. at a junction of two media, and this 

 may be called the E.M.F. of contact, without any view as to 

 its origin or magnitude. 



This contact E.M.F. may be caused in a chemical or in a 

 physical manner. It is generally admitted, I believe, that 

 the E.M.F.s concerned in thermoelectricity are caused in a 

 physical manner. It is generally admitted, I believe (I think 



