ELECTRICAL CONDUCTIVITY OF CERTAIN SALINE SOLUTONS. a7 
pair to a very small fraction (gg5o50) Of a Grove’s cell. Their intensity was 
measured by an electrodynamometer. In this way KoHLRAuscH and NIPpPpoLpT 
determined the resistance of solutions of sulphuric acid, and found that at a 
temperature of 22° C. the maximum conductivity is reached when the specific 
gravity is from 1:20 to 1°25, a result which agrees fairly with those of Becker 
and others. 
The method of Paatzow (Berlin, Monatsbericht, 1868; also Pogg. Ann., 
cxxxvi., 1869) was very ingenious. He employed for electrodes two pieces of 
pure zinc, which were placed in the bottom of glasses filled with saturated solu- 
tion of sulphate of zinc. These two glasses were connected together by a siphon 
filled with the liquid whose resistance was to be measured. It is well known 
that pure zinc electrodes do not become polarised in a solution of sulphate of 
zinc. Hence this method avoided polarisation, provided that none took place at 
the junctions of the two liquids. We are not aware that any experiments have 
been made to determine whether this is possible. It might form an interesting 
subject of inquiry. PAaAtzow’s method permitted that sufficiently low electro- 
motive forces might be used to avoid electrolysis. The diffusion of the two 
liquids must have been a source of error, especially as the resistance of mixtures 
is totally different from that of their components (v. page 67). . 
These notices are sufficient to show how much importance has been attached 
to this subject. The various modes of definition, both of the solutions and of 
their resistance, and also the variety of temperatures adopted by the above 
experimenters, render comparison of their results extremely difficult. The 
attempt, however, has been made by WIEDEMANN in his “ Lehre vom Galvan- 
ismus,” vol. i, part 1. He finds great discrepancies in many cases, which are 
probably due to the disadvantages of the several forms of experiment. We 
think that these disadvantages are avoided in the following mode of testing, a 
mode which, so far as we know, is considerably different from any that have 
hitherto been made use of. 
The electrical resistance of a substance is easily measured, if it either does 
not act at all as a producer of an electric current, or produces a constant 
one. A wire is an example of the first class ; the cell of a galvanic battery is an 
example of the second. The resistance of the first is generally best measured 
by the “ Wheatstone bridge ;” that of the second by the electrometer, by 
“shunting” the current through a known metallic resistance. But the solu- 
tions under examination fulfilled neither of these conditions: not the first. 
because of the polarisation of the electrodes; and not the second, because the 
polarisation disappears (after the polarising current is stopped) far too rapidly 
to allow of such a measurement. We found, however, that polarisation does 
not attain its maximum whenever a current is made to pass through the liquid. 
in fact, that at the moment the circuit is completed there is vo polarisation. It 
