Ml*. I. B. Cooke on the Measurement of Chemical Affinity. ^3' 



electrolyzecl. The wires weiv? so adjusted, that it was easy ii*i'«^ 

 stantaneously to disconnect the battery, and to join either pla- 

 tinum plate to the resistance tube. As soon as bubbles of 

 oxygen began to form on the anode platinum, its junction with 

 the instrument and copper plate was eifected. The swing of the 

 needle was now 5° when the current was passed through fifty- 

 nine divisions of the tube, giving an intensity = 5 (59 H- 4) = 315. 

 The swing could be reproduced several times undiminished with- 

 out reconnexion with the battery. The total secondary current 

 between the two platinum plates =5(114 + 4) = 590 ; but this 

 current was more inconstant than the former, and began imme- 

 diately to diminish. When the cathode platinum plate was con- 

 nected with the galvanometer, and the copper plate with the 

 resistance tube, the intensity was 5(51 4-4) =275, which equals 

 the difference of the two former numbers ; but was very incon- 

 stant, and could instantly be reduced almost to zero, by shaking 

 the platinum plate so as to dislodge all the bubbles of hydrogen. 



The anode platinum plate was found to receive its maximum 

 polarization from the current of two cells of Smee^s battery. 

 The current of six cells did not increase it. The cathode received 

 its maximum polarization from three cells, and after being 

 thoroughly covered with hydrogen, could receive no increase of 

 force. 



The polarization of the cathode plate therefore clearly resulted 

 from the coating of hydrogen with which it became covered, but 

 the anode received its greatest charge before a single bubble of 

 oxygen was formed ; and indeed, since oxygen was already pre- 

 sent in solution in the water, its additional production by elec- 

 trolysis was not likely to produce an increased effect. '' 



This definite amount of polarization of the anode plate would 

 be accounted for, if its surface could be supposed to contract 

 from the atmosphere, in its ordinary condition, a covering of 

 matter possessing an affinity for oxygen. The current would of 

 course be affected by this covering at its commencement, as by 

 an electro-positive element; but as the covering became eaten 

 away by the oxygen determined to its surface by the force of the 

 battery, the purified platinum would begin to act simply as con-« 

 ductor, and wholly indifferent as to afiinity. . ./ ' 



That platinum and other substances do contract such a' coijer- 

 ing. Dr. Faraday has proved in his experiments on the catalytic, 

 deflagration of explosive gases. He has also shown, that expo-- 

 sure at the anode of a battery to the action of nascent oxygen, is 

 identical in its results with a mechanical purification of the sur-' 

 faceofplatinmn. ' '^^j':^'!;::'^^ 



This covering is instantly destroyed by immersi&ri''iii stron^'^ 

 nitric acid ; and it is to this fact probably, more than to the 



