Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1958. The general techniques of voltammetry with continuously varying potential have been applied to a differential method in which two electrolysis cells are used, each with a hanging mercury drop electrode. Using this method it is possible to determine lO/sup -6/ solutions with increased precision, and it is also possible to analyze solutions as dilute as 10/sup -6/ without removing oxygen from the electrolysis cells. Mixtures can be analyzed easily by adding individual components of the mixture to the reference cell. (auth)