214 Mr. A. P. Laurie on the Electromotive Force of 



phate of zinc is formed, and sulphate of alumina decomposed 

 during the passage of a current, and that an enormous ab- 

 sorption of heat takes place to enable this reaction to go 

 forward. 



If we believe that we are dealing only with metallic alu- 

 minium, sulphate of alumina, sulphate of zinc, and metallic 

 zinc, then we must make the above assumption, and un- 

 doubtedly chemical analysis would confirm such a belief. 



I think, then, I have said sufficient to show that we have 

 here a point of some importance,^-that if we have only 

 the above substances to deal with, Dr. Wright has made a 

 surprising discovery. 



It was to test whether we really had these substances 

 present that I have made the following experiments. 



Before describing the experiments, however, I must say a 

 few words about aluminium itself. It is a metal with some 

 very peculiar and contradictory properties. Though having 

 a very high heat of combination with oxygen, it does not 

 tarnish in the air or decompose water at ordinary tempera- 

 tures, nor is it soluble in nitric acid. If used as the electrode 

 at which oxygen is set free by the passage of a current 

 through dilute sulphuric acid, it opposes a very considerable 

 resistance to the passage of the current. This resistance has 

 been called "polarization," and stated as high as six volts. 



On amalgamating the metal (which can only be done by 

 immersing first in strong caustic) it becomes capable of de- 

 composing water ; and its oxidation in the air is so rapid that 

 if left at rest it becomes covered with a growth of oxide in a 

 few minutes, resembling some fungus growths in appearance. 



Now all these properties seem to me to be very easily ex- 

 plained by supposing aluminium to become rapidly coated 

 with a film of oxide, whether in air or water, which, being 

 very insoluble, protects it from all further action, and practi- 

 cally for chemical purposes converts an aluminium plate into 

 an aluminium-oxide plate, unless some solvent of the oxide is 

 present. 



Starting with this view, I prepared a solution of aluminium 

 sulphate and of zinc sulphate, enclosing one in a porous pot, 

 and immersed a zinc rod in the zinc sulphate and an alumi- 

 nium wire in the aluminium sulphate. I did not note the 

 strength of the solutions, as the experiments were merely 

 qualitative. 



I obtained a deflection on connecting this cell with the 

 electrometer, amounting to about *54 volt in the same direc- 

 tion as that obtained by Alder Wright. On removing the 

 aluminium wire, cleaning it with sandpaper, and immediately 



