106 Mr. J. P. Joule on the Electric Origin 



Intensities of the Affinities which unite bodies tsoith Oxygen. 



34. In order to ascertain the intensities of galvanic arrange- 

 ments, we may either use a galvanometer furnished with a short 

 and thick wire, or with a long and thin wire (within certain limits 

 (l^.)). In the former case the calculations must be conducted 

 on the principles of Ohm; in the latter it is only necessary to 

 take care that the resistances of the pairs under comparison are 

 pretty nearly equal, in order that the deviations of the needle 

 may be depended upon in calculating the intensity of the cur- 

 rent. I have adopted the latter plan on account of the supe- 

 rior facilities which it presents. 



35. Affinity of zinc for oxygen. — ^l^rom (32.) we have the in- 

 tensity of the action of zinc = 0°*056 Q ; and the intensity re- 

 quired for the electrolysis of water = 0°"029 Q. Hence 

 29 : 56 : : 1, the affinity of hydrogen, : r93, that of zinc for 

 oxygen. 



36. Affinity of iron for oxygen, likewise obtained from (32.), 

 is 1-27, for 268 : 340 : : 1 : 1-27. 



37. Affinity of potassium for oxygen. — Twenty grains of 

 potassium were combined with about ten ounces of mercury. 

 The amalgam was poured into a wooden cup, into the bottom 

 of which a copper wire connected with the galvanometer (5.) 

 had been let. At about half an inch above the surface of the 

 amalgam I secured a piece of platinum, also in connexion with 

 the galvanometer. On pouring dilute sulphuric acid into the 

 cup the needle was deflected 74° (= 0°*05 Q) during three 

 successive minutes, but the local action of the amalgam was so 

 vigorous that at the end of this interval of time most of the 

 potassium was dissolved, and the needle declined very fast. 

 On treating 20 grains of zinc in precisely the same manner, I 

 had a deviation of 49° ( = 0°'0152 Q). Hence 



Jc — h = 0-05, 

 and z — h = 0-0152, 



whence \52k - 500 z - S^Sh. 



But from (35.), z = 1*93, and /< = 1 ; therefore k, the affinity 

 ofpoassium, = 4'06. 



38. It is necessary, however, to pay attention to the circum- 

 stances under which the experiments were made, in order to 

 obtain correct ideas concerning the above intensities of affinity. 

 The increase of the intensity of the voltaic apparatus by heat 

 is by no means great; and as all the experiments were con- 

 ducted at common temperatures, no regard need be paid to it. 

 But then the intensities of affinity were obtained by comparing 

 currents which had been produced under peculiar circum- 

 stances with regard to the condition of the elements of the gal- 

 vanic arrangements : in one case the hydrogen was evolved in a 



