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admire the ingenuity of the methods which he employed, it is 
impossible to speak of them as having been productive of positive 
results, and unwarrantable to describe them as methods which have 
increased our knowledge of this department of Physics. Hence 
we did not mention his whole plan, nor refer to all the methods by 
which he endeavoured to carry it out, but confined our attention to 
that part of it which he had successfully accomplished. If we are 
justified in having thus restricted our remarks to really fruitful work, 
we must also be justified in having given as his only precautions 
against polarisation, those which he adopted in his absolute 
measurements of the sulphate of zinc. Two sentences will show 
what these were : — “ If we only knew a combination of liquid and 
electrodes, in which the electric current produces neither polaris- 
ation nor new resistances, it could be treated, so far as the 
measurement of resistance is concerned, exactly as a solid body.” * 
Such a combination is easily obtainable, for “ through du Bois- 
Keymond’s investigations it is known that amalgamated zinc 
electrodes are not polarised in concentrated solutions of zinc 
vitriol.”f The only necessary precaution is thus found; and, as 
might be expected, one looks in vain for an account of any others. 
Of course he made his solutions as pure as possible, excluded air, 
arranged his tube so that it would always be full of liquid, and kept 
his electrodes in the same positions ; but these were not directed 
immediately against polarisation. His only precaution, in fact, 
was the only necessary one, viz., the use of non-polarisable 
electrodes. 
In describing Professor Beetz’ work, however, we fell into an 
error by assuming that pure zinc electrodes are not polarisable in 
solutions of zinc vitriol, and, therefore, regarding it as remarkable 
that he should have taken the unnecessary trouble of amalgama- 
tion ; and the same error is seen, as he points out, in our reference 
to Paalzow’s work, in which we say that he used pure zinc 
electrodes, while in reality he amalgamated them. This is, I 
think, the only “ Wissensfehler” of which we can be convicted. 
As an error it is, of course, to be regretted. But its comparative 
triviality is shewn by the fact that it not only has no influence on 
our own work, but does not even affect our criticism of either Pro- 
* Pogg. Ann. cxvii. p. 3. + Ibid. p. 6. 
