144 Dr. J. Ei-skine-Murray. 



The copper plate was now gently heated with a bolt until it was hot 

 enough to melt paraffin-wax ; the temperature, ahout 50 C., required 

 for this is not sufficient to cause sudden permanent change of the 

 copper surface (see 74). Paraffin-wax was then poured on, and 

 the plate was filed with its own file while covered with molten wax. 

 Thus the fresh surface exposed by the filing came directly into 

 contact with the wax. More wax was poured on and the filings 

 drained off, the plaife remaining well covered with wax all the while. 

 Ifc was then allowed to cool, and gave with the bare zinc plate 



+0-555 volt, 



zinc being positive as before. Thus the change due to substituting 

 paraffin-wax for air next the copper is not more than + O'lOO volt. 

 I now waxed the zinc in exactly the same way. When it was quite 

 cool it gave with the waxed copper 



+ 0-602 volt, 



showing that waxing the zinc had raised its potential 



+0-047 volt. 



So, on the whole, the substitation cf wax for air on both copper and 

 zinc had only decreased their mutual potential by 



+ 0-053 volt 



and the potential of the waxed plates remained nearly constant for 

 several hours. The changes due to waxing the plates as given above 

 were confirmed by the independent comparison of each plate with the 

 standard gold plate. It does not follow that even the small changes 

 which did occur were due solely to the substitution of wax for air, for 

 they may have been caused by slight changes in the surface on account 

 of the filing. 



80. An aluminium plate was coated with wax in the way 

 described in 79, a knife being used to scrape the surface under the 

 molten wax. When cold, this waxed plate gave with a bare zinc one 



+ 0-36 volt, 



which is about the usual value for bare aluminium and zinc. The 

 removal of the air had therefore not appreciably altered the potential. 

 81. I now warmed the waxed aluminium slightly. Its potential 

 with the zinc varied as follows : 



47 C +0-41 volt. 



35 C +0-37 



28 C +0-35 



This gives a variation of about 



0'0032 volt per degree centigrade, 



