2i>8 



Mr. J. Brown. 



[Nov. 18, 



Copper of cell to zinc quadrant — 



Right, 76 ; left, 75 ; right, 74'5. a= 150-25. 

 Copper of cell to copper qnadrant — 



Right 19-5; left, 11-5; right, 20*5. b= 315. 



Whence p = 0*65 Daniell. 



Owing to the nature of the apparatus, extreme accuracy in the 

 results as absolute quantitative measurements is not to be expected. 

 They are meant in most cases more for comparisons among them- 

 selves. 



10. A number of experiments were made to ascertain the rate of 

 decrease of difference of potential of the films on copper and zinc in 

 air due to gradual tarnishing of the metals. The curves corresponding 

 to two of these with zinc of different degrees of purity are given in 

 fig". 2, where ordinates are in decimals of the electromotive force of 

 one Daniell, and abscissae represent the time elapsed afteT cleaning 

 the metals with glass paper. Ten more ohservations were made on 

 the pair of copper and redistilled zinc giving curve A. The remainder 

 of the curve has the same character, falling in 61^ hours to 64 

 Daniell. 



The object was to estimate the difference of potential due to the 

 clean metals by carrying the curve back to the time at which they 

 were cleaned, and compare this with the difference of their heats of 

 combustion in oxygen as compared with the heat equivalent of the 

 standard cell. The result agrees fairly well with the thermocbemical 

 data of J. Thomsen (' Wiedemann, Beiblatter,' 1880, Nos. 7 and 8). 



We are met, however, with the fact that with many other pairs of 

 metals in air we do not find the same agreement, although in most 

 cases there is a more or less near approximation. This I believe is 

 due to two causes : — 



(1.) We cannot tell the exact nature of the chemical process going 

 on at the surface of the metals, whether it be simply an oxidation ; 

 which particular oxide of the metal is formed, or if the other gases, 

 ammonia, carbonic acid, &c, take any part in the process, nor do we 

 know exactly the physical state of the film. 



(2.) We do not know how much protective effect the already 

 formed coating of oxide, &c, exercises on the metals. 



11. The comparatively small and often irregular difference of 

 potential observed when experimenting with copper and iron may be 

 due to the formation on the latter, immediately after cleaning, of a 

 highly protective oxide coating, producing a more or less passive 

 state analogous to that which this metal assumes under other well- 

 known conditions. The rapid tarnishing of lead may also produce a 

 similar effect supposing the oxide formed be of a very coherent and 

 protective nature. 



