Abstract
RECENT letters from Dobinski1 and Nelson2 report the rapid appearance of oxide upon unheated copper and iron on exposure to air. This oxide, which doubtless causes the rapid change in the behaviour of these metals to silver and copper nitrates respectively3, can be estimated by measuring the number of millicoulombs needed for its cathodic reduction. The principle was formerly used to measure silver iodide films4, and gave numbers agreeing with both gravimetric and iodometric methods. For oxide films it has yielded results in reasonable accord with optical5 and gravimetric methods (see accompanying table), and consistent with early chemical determinations made on the films after stripping6. Former disagreement between gravimetric and optical methods has been largely due to neglect of invisible films, or to different meanings attached to the term thickness as applied to a non-uniform film on an uneven surface.
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