REVERSIBLE AND IRREVERSIBLE SYSTEMS UNDER INFLUENCE OF LIGHT. 339 
we have at the same time the most efficient and reliable method of distinsruishinfr 
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between constant cells showing no polarisation and combinations showing polarisation. 
If a system shows no polarisation, and the above conception of the curve be true, 
there ought to be, during the induction period, only a gradual increase of the E.M.F. 
till the maximum is reached, and no drop, and similarly only a gradual diminution of the 
E.M.F. should be observed when light is removed and the system returns to its state 
in the dark. This is now the second kind of curves we actually obtain with a series 
of other systems giving constant reversible cells. A careful consideration of the 
reactions going on in these systems, under the action of light, shows also why they 
must be reversible constants. 
As a result of this varied and very complicated research, which took over three years, 
we find that under the action of light we have a region of galvanic cells which is just 
as wide, just as varied, though often inaccessible to measurement, but regulated by 
just as solid principles, as ordinary galvanic cells. As in the case of ordinary galvanic 
cells, we have, under the action of light, reversible and irreversible cells, constant and 
inconstant cells ; constant cells, the electrodes of which are “ reversible in respect of 
the anion, constant cells, the electrodes of which are “ reversible in respect of 
the cation ”; we have cells showing polarisation owing to. the gases separating 
on the electrodes, and cells showing polarisation owing to the electrodes changing 
the nature of their surfaces, &c. The chemical reactions (and chemical equilibrium) 
occurring in these systems and their individual nature are now perfectly clear to us 
and can be measured quantitatively, though the amounts of the transformations, as 
will be seen later on, are so exceedingly small that there is not the remotest possibility 
of even detecting the products of reaction by any chemical means, 10~ 9 ampere 
indicating a decomposition of 1 gramme electrochemical equivalent in 30,000 years. 
The true meaning of the observations of Becquerel and Minchin is now apparent. 
“The very curious inversion of the current” observed by Minchin with “ Ag plates 
immersed in water containing eosine in solution ” (p. 211), “ Ag plates immersed into 
alcohol containing naphthalene red ” (p. 212), with “bismuth in water” (p. 213), with 
tin plates in alcohol (p. 216), with his “sensitive” cell, consisting of a tin plate 
acted upon by nitric acid, and a tin plate in methyl alcohol (p. 222), is evidently in 
each case due to the fact that all these systems, as now clearly appears from their 
composition, must form inconstant cells exhibiting polarisation. 
The reason why Becquerel with Ag, BrAg and IAg plates found a current 
sometimes in one direction, sometimes in another, is not “ the thickness of the 
layer, but the kind of combination of the heterogeneous system, of which the given 
plate forms only a part. An Ag-AgCl plate in NaCl solution gives a constant cell, 
reversible in respect of the anion; its current must, as will be shown later on, flow 
from the plate in the dark to the plate in the light. If, however, the layer of AgCl is 
very thin, such a system will soon transform under the action of light into the 
system (Ag in light, NaCl solution, Ag-AgCl in dark) which is inconstant, showing 
2x2 
