506 



KEroRT — 1 884. 



■which of them we are to consider as most applicable to the case of the Volta 

 experiment. Perhaps one should take the most common or stable oxide ; 

 perhaps, seeing that no combination is supposed actually to occur, and 

 since the metal is, so to speak, in excess, it is most reasonable to take 

 the lowest oxide which the substance will form. It may be that the 

 data are not known for this ; it may even be that they have only been 

 < ibtained for the hydrate instead of for the oxide. 



I must therefore do the best I can, and quote several numbers 

 wherever there is obvious doubt. I imagine that J. Thomsen's are the 

 most reliable when they arc available. 



But it must be remembered all through that since it is only the 

 tendency to chemical action which is the cause of the Volta effect, whereas 

 combination heats are obtained by permitting or causing the combination 

 to actually occur, the numbers obtained by calculation are not likely to be 

 quite right ; and they may be expected to err on the side of excess, the cal- 

 culated number being higher than the actual value if directly observed. 



