Page:Text-book of Electrochemistry.djvu/220

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This idea waa firat suggested by Helmboltz (/), and was afterwards taken up by Lord Kelvin (W. Tliomson) (..'), and is known as the Thomson rule.

Several attempts, notably by Raoult and J. Thomsen, have been made to confirm the Thomson rule, and it has been found that in the cases investigated the experimental values agree well with the theory. Thus, Thomsen {■)) obtained the following values for the electromotive forces of several elements, 1 dll. being taken as unit : —

��1 1 CiiSO. 1 Cu

�2606r.

�|Cd. '..

�fi-.:::

�270*»

�l-(w

�1-OC

�+ 7H,0 1 C ,

�C. . .'.

�+ 7H.0IC.

�leaio

�C. . ..

�l>9<)

��Zn I B Zn B Zn B Zn B ZnlB Cu B Cu B Cu|h Fo IF

��When concentrated nitric acid was used, Thomsen assumed that nitrogen peroxide (NaOj) was formed, and remained dissolved in the liquid ; when more dilute nitric acid (HKOg + THjO) was used, he assumed that nitric oxide (NjOs) was produced. Consequently there was a different heat evolved when these were employed. In the chromic acid cell it was assumed that chromic oxide was formed, and in the ferric chloride cell that ferrous chloride was produced.

Criticism of Thomson's Rule. — ^When the Thomson rule was more fully examined difficulties arose. The cause of the deviations was sought for in so-called secondary processes by which heat is evolved, but which were supposed to have no influence on the electromotive force.

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