in Cells for Direct and Alternate Currents. 337 



microscope. Not much information, however, is to be gained from 

 this. The skin over the plates is, however, seen to be full of minute 

 cracks in everj direction, giving the impression of a dried gelatinous 

 pellicle; not an unexpected appearance if the plate had been covered 

 when wet with a thin coating of the gelatinous aluminium 

 hydroxide. The analysis of the film over the metal shows it to 

 consist of basic aluminium sulphate. The origin of this may of 

 course have been the formation in the first place of aluminium 

 hydroxide which subsequently reacted with some of the alum solu- 

 tion to give the basic compound. How far the formation of this 

 may have anything to do with electrolysis would, I should think, 

 be difficult to say without further and more elaborate experiment, 

 and it must be remembered that a similar coating can be obtained 

 on an aluminium plate by immersing it in an alum solution and 

 leaving this freely exposed to the air. 



" Yours sincerely, 



" HERBEKT JACKSON." 



I have tried an experiment which I think shows that a film formed 

 on aluminium by first being placed in contact with an alum solu- 

 tion and then exposed to the air, gives the same effect as a film 

 formed in the cell when a current is passed from the aluminium to 

 a carbon plate through the electrolyte. Two similar bright alu- 

 minium plates were prepared with carbon plates on either side of 

 them, the area of aluminium in the electrolyte being 8J square 

 inches. The electrolyte consisted of a saturated solution of potash 

 alum in water. One plate was left in the solution for seven hours 

 with no current passing, and then exposed to the air for 16f hours. 

 The other plate was submerged in the fluid and immediately read- 

 ings were taken of the current passing and potential difference in 

 volts between the aluminium and carbon plates. The effect in this 

 case was exactly what was previously observed. That is to say, the 

 current being maintained constant at 0'055 ampere by about 2000 ohms 

 being inserted in circuit with the cell across about 110 volts, 

 the potential difference immediately passed from a small negative 

 value through zero, and after three minutes and forty minutes, had 

 respectively the values 1'8 and 2'27 volts ; the temperatures being 

 12^ and 15^ C. The test was continued. For the next 5 hours 

 the current was 0'204 ampere, it was then dropped to 0*055 ampere 

 and kept at this value for 16J hours. At the end of this time the 

 potential was 10 volts with 0'055 ampere, and 28*6 volts with 0'163 

 ampere, the temperature being 15 C. 



The other plate was submerged in its solution, the potential differ- 

 ence and current being immediately noted as before. The results in 

 this case show that the potential, with 0'054 ampere, rose immediately 



