Mr. I. B. Cooke on the Measurement of Chefriical Affinity. 95 



riband^ and also connected with it by its wire through the gal- 

 vanometer. The copper solution should be thoroughly stirred 

 up by the tube previous to experiment, and the room be kept 

 during experiment as nearly as possible at a uniform tempera- 

 ture. With these precautions, measurements may be taken 

 again and again without the slightest variation. Any great 

 changes in the temperature will of course necessitate a correction 

 in the allowance for the resistance of the galvanometer coil. 



Table of the Affinity of various Substances for Oxygen. 



Zinc — copper (standard) . =1 



Hydrogen =2*36 



Zinc =2-23 



Potassium =3*13 



Sodium =2-91 



Iron =1*85 



Tin =1-75 



Lead =1*7 



Bismuth =1-29 



Antimony =1'29 



Copper =1-25 



Silver = -85 



These affinities do not appear to be affected by changes of 

 temperature between the range of 50° and 212° F. 



It is almost impossible to obtain mercury perfectly free from 

 traces of impurity ; and however small may be the quantity of 

 any substance more electro-positive than itself which may be 

 dissolved in it, the mercury usurps to the full the affinities of 

 that substance. This property renders its own affinity for oxygen 

 difficult to determine, but has been made use of to ascertain 

 those of potassium and sodium, the action of which metals upon 

 water is otherwise too violent to admit of satisfactory experiment. 

 A fluid amalgam of either of these metals decomposes water 

 slowly, especially if the surface exposed be small compared with 

 the bulk of the "^amalgam, and a deliberate experiment is thus 

 permitted. 



It will be seen from the table, that hydrogen has a higher 

 number assigned to it than is given to zinc. Why then, it may 

 be asked, does zinc so easily decompose acidulated water ? The 

 truth is, that the zinc is retained in its salts generally by a force 

 fully as strong as that which combines it with oxygen alone, 

 while hydrogen is held in acidulated water with a considerably 

 slighter affinity than in pure water. Pure zinc will not decom- 

 pose pure water if atmospheric air or oxygen be not present. 



The principle developed in this paper is probably applicable 



