742 rEPoRT—1885. 
by Lord Rayleigh ; for this substance, therefore, we know that it is true 
to a high degree of accuracy. For other substances our knowledge is at 
present only approximate, the main difficulty in its verification lying in 
the prevalence of secondary actions and the confusion they cause. So 
great is the influence of these actions that in some cases an electrolyte 
has been asserted to conduct only metallically, and not to be decomposed 
at all. This has been said, for instance, of the fused iodide and chloride of 
mercury. A strong current may be sent through these fused salts with 
carbon electrodes and no products of decomposition shall appear, because 
they dissolve and diffuse throngh the lhquid with such surprising velocity. 
J. W. Clark! has proved this to be the explanation, and has succeeded 
in getting off a good supply of mercury, and of iodine or chlorine, by 
separating the electrodes by a sufficiency of intervening porous material, 
which, whether it retard true diffusion or not, certainly hinders con- 
vection. 
But of course such an experiment has no exact quantitative signification 
until every trace of recomposition is avoided, and the theoretical amount 
of free ions obtained. And this is just the difficulty ; for even in simple 
acid water, if any air be dissolved in the water it is well known that an 
insufficient supply of hydrogen comes off, while if the water is not satu- 
rated with oxygen, the supply of oxygen must be deficient by reason of 
solution. 
If oxygen be estimated by volume it will be apparently deficient for 
another reason ; but the better way of estimating it is by loss of weight, 
which gets over the ozone difficulty. 
Helmholtz has taught us how to get over the dissolved air difficulty, 
by making a most ingenious air-free cell, described in the Faraday Lec- 
ture, 1881; and he shows that in such a cell polarisation is produced by 
infinitesimal currents, and no permanent leak goes on through the cell 
until the applied E.M.F. attains a certain value. This is a pertectly valid 
proof that no trace of metallic conduction exists in air-free acid-water, 
and that accordingly for this substance Faraday’s law 1 is true. 
We have already expressed the law in several forms of words. Helm- 
holtz expresses it as follows: ‘Through each section of an electrolytic 
conductor we have always equivalent electrical and chemical motion.’ 
And if this fact of equivalent electrical and chemical motion be expressed, 
as Ampére very naturally expressed it, by calling it a convection of elec- 
tricity by the moving atoms of matter, we may state Faraday’s law 1 thus: 
Electrolysis is a kind of electrical convection rather than conduction, 
each atom carrying a charge with it; and the charge conveyed by every 
atom of a given substance is the same. 
Obviously a vitally important statement if the slight amount of 
hypothesis involved in it is legitimate, as I fully believe it to be; and it is 
the one virtually adopted by Clerk Maxwell in his treatise. ; 
Law 2 asserts that when a current is sent through a series of different. 
substances, the mass of each substance liberated (or decomposed, or 
dissolved, or whatever it is that happens to it) is proportional to its 
ordinary chemical equivalence : or, that the amount of any substance acted 
upon during the passage of a given quantity of electricity is equal to its 
molecular weight divided by its atomicity, or, more explicitly, its mole- 
cular weight divided by the number of bonds which under the particular 
1 Phil. Mag., November 1885, 
