I* 



SCIENCE AND PRACTICE. 325 



the sum of the other two differences. Imagine a scale on 

 which the distances from a common point, o (Fig. 152), to 

 certain heights, a b c, represent the electro-positive polarisa- 

 tions of three metals, A, B, C, when plunged into 

 water ; then the distance a b y which is the difference 

 between their electric conditions, will be the expres- 

 sion of the electro-motive force of a galvanic pair 

 composed of the metals A and B, and the distance 

 b c will be similarly the electro-motive force of a 

 pair of B and C. A pair of plates of the metals 

 A and C will evidently have an electro -motive force, 

 a c, equal to the sum of the two electro-motive forces 

 a b and b c. 



This will also be observed in the following table ; 







152. 



the sum of the two first values being very nearly equal to the 

 third of the same series. 



A. Two metals and one fluid. 



(The unit in which the electro-motive forces are expressed is 

 that of a Daniell's Element.) 



I. In sulphuric acid (sp. gr. = 1'838) 1 part, water 49 parts. 



1. Zinc Tin ............................................. 0-409 



Tin Copper .......................................... 0-410 



Zinc Copper. ......................................... 0'824 



2. Zinc Copper .......................................... 0-837 



Copper Silver ................................. ..... 0-214 



Zinc Silver ..... ....... . ...... .... ...... '..' ....... .... 1-053 



3. Copper Mercury ................ .... ................ 0*356 



Mercury Platinum ................................ 0*231 



Copper Platinum ...................... . ............. 0-604 



II. In nitric acid (sp. gr. = 1'222) 1 part, water 9 parts. 



4. Zinc (amalgamated) Copper .................... 0*882 



Copper Platinum.... ................................ 0*616 



Zinc (amalgamated) Platinum . ................. 1-495 



III. In muriatic acid (sp. gr. = 1-113) 1 part, water 9 parts. 



5. Zinc (amalgamated) Copper ..... . ........... 0*788 



Copper platinum ........................... ......... 0*743 



Zinc (amalgamated) platinum ; ................ 1*537 



