Wollaston slide rule of chemical equivalents, 1814.
© Science Museum / Science & Society Picture Library
William Hyde Wollaston (1766-1828) was a pioneer of powder metallurgy and discovered two new metals, rhodium and palladium. Wollaston developed this slide rule to enable calculations of chemical proportions (relative weight of reactants), using a logarithmic scale of chemical equivalents. Like most early 19th century chemists, Wollaston used equivalents rather than atomic weights in an attempt to avoid the various problems asociated with the latter. Compounds and elements are marked on the outer scale.