Description
In chemical reactions, given compounds always combine in the same weight ratios. In the early nineteenth century the weight ratios of combining substances in chemical reactions were studied extensively and the term ‘chemical equivalents’ was used to defined the proportions in which the common chemical substances (either elements or molecules) combined.

In 1814 the Englishman William Hyde Wollaston proposed using a slide rule from which known chemical equivalents could be read. On the chemical slide rule distances are proportional to the logarithms of the combining quantities, making computations straightforward to find what weights of different chemical substances combine to produce a given product weight (or conversely how much of each material results from decomposition).

Through the nineteenth century the accepted values of chemical equivalents were in a state of constant flux. New elements were being discovered (19 were added during the first quarter of the century) and research continued on the amounts consumed in various chemical reactions. Wollaston’s 1814 slide rule used oxygen = 10 as its base value, the figure chosen as a matter of convenience. Wollaston was yet to be convinced by Dalton’s atomic theory, calling it “purely theoretical” and, therefore, of no use to the “formation of a table adapted to most practical purposes”. Not desirous of “warping my numbers according to an atomic theory” Wollaston thus made oxygen the decimal unit of his scale because oxygen was the most common reactant in numerous important chemical reactions and the number ten was a simply figure to manipulate in calculations.

This slide rule by Martin Ehrmann takes after Wollaston’s and was probably produced within 15 years of Wollaston’s original. It too used oxygen = 10 as its base unit, the rule listing 175 entries for elements and compounds in total (compared to Wollaston’s original 94). Ehrmann was professor of chemistry at the universities of Vienna and Olomouc (in Moravia), a founder of the first pharmaceutical association and journal in Austria, and author of a number of books.
03/01/2008
Created by: Joshua Nall on 03/01/2008