of Edinburgh, Session 1874-75. 
547 
lessor Beetz or of Paalzow. In the case of the former, we attri- 
buted to the electrodes which he used the same property as he did 
himself, and in the case of the latter we attributed to those which 
we described him as having used, the properties which belong to 
the ones which he really did use. Nevertheless, Professor Beetz 
thinks that we did not understand Paalzow’s method.* “ The 
authors,” he says, “ call this method 4 very ingenious/ 4 Curiously 
enough ! ’ for they did not understand it at all. If they only 
had, how ingenious it would then have appeared to them.” 
He comes to this conclusion because he thinks that, had we had 
a full knowledge of Paalzow’s apparatus, we could not have 
criticised it as we did. All that is necessary, therefore, for me to 
prove, is that the items of description which we, for shortness’ sake, 
omitted, do not necessitate a change in our criticism. Paalzow’s 
apparatus, according to our description, consisted of two glasses 
filled with sulphate of zinc solution and joined by a bent tube con- 
taining the solution under investigation, the electrodes dipping 
into the zinc sulphate. Professor Beetz thinks that we had neither 
grasped the idea that the tube did not open into the glasses con- 
taining the vitriol, but into porous clay vessels which contained the 
same liquid as the tube ; nor understood the meaning of his having 
made two or more measurements of the same solution in tubes of 
different lengths. Do, then, these facts destroy our criticism ? 
We said that diffusion of the liquids must be a source of error, and 
that polarisation at the surface of junction might be. These 
dangers are not excluded even when the clay vessels are used. Dif- 
fusion may not go on so rapidly, but it still goes on ; the mixture 
which must take place constantly changes the conductivity, and 
(as our experiments t shew) possibly to a great extent. Moreover, 
there are still surfaces of contact, notwithstanding the intervention 
of the clay vessels, the only difference being that instead of one 
large one there are numerous small ones; and there is still, 
therefore, the danger of polarisation J without the possibility of 
eliminating its effects by calculation. The clay vessel not only 
* Monatsberichte der Berliner Akademie, 30. Juli, 1868. p 486. Pogg. 
Ann. cxxxvi. 1869, p. 489. 
t Trans. Roy. Soc. Edin., vol. xxvii. part 1, 1872-73, p. 67. 
t Monatsberichte der Berliner Akademie, 17 Juli, 1856, p. 1. 
VOL. vui. 4 R 
