(b. Umstadt [near Darmstadt], Germany, 15 April 1833; d. Glasgow, Scotland, 9 February 1892)
Dittmar analyzed samples of seawater collected during the voyage of H.M.S. Challenger (1872–1876). He originally qualified in pharmacy, but in 1857 he went to work in Bunsen’s laboratory at Heidelberg. There he was befriended by Henry Roscoe, who invited him to England; and when Roscoe became professor of chemistry at Owens College, Manchester, Dittmar accompanied him as his assistant. From 1861 to 1869 he was chief assistant in the chemical laboratory at Edinburgh University.
After spending the next three years in Germany, where he lectured in meteorology at an agricultural college near Bonn, Dittmar returned to Edinburgh in 1872. In 1873 he was appointed assistant lecturer at Owens College but the year after became professor of chemistry at Anderson’s College, Glasgow, where he remained until his death.
Dittmar’s “Report on Researches Into the Composition of Ocean-Water” (1884) was the result of six years’ work on the Challenger samples. He improved on existing methods of determining the major constituents of marine salt and confirmed Forchhammer’s discovery (1865) that although the salinity of seawater varies from place to place, the ratios of the principal constituents to each other remain almost constant. This showed that calculations of salinity, usually made from measurements of specific gravity, could be done as well, if not better, by determining the weight of chlorine in a sample and multiplying it by a constant factor of 1.8058, the ratio of total salt to chlorine. Dittmar also studied the absorption of gases in seawater and established the proportion of bromine to chlorine.
Dittmar’s other work included research on the atomic weight of platinum and on the gravimetric composition of water, for which he was awarded the Graham Medal of the Glasgow Philosophical Society. He was elected a fellow of the Royal Society of Edinburgh in 1863 and of the Royal Society in 1882.
I. Original Works. Dittmar’s “Report on Researches Into the Composition of Ocean-Water” was published in Report on the Scientific Results of the Voyage of H.M.S. Challenger: Physics and Chemistry, I (London, 1884). He also wrote a number of scientific papers and textbooks. See Poggendorff, III, 365–366; IV, 333.
II. Secondary Literature. Obituaries of Dittmar are in Nature, 45 , no. 1169 (24 Mar. 1892), 493–494; and Proceedings of the Royal Society of Edinburgh, 20 (1892–1895), vi–vii. His contribution to oceanography is described in J. P. Riley and G. Skirrow, eds., Chemical Oceanography, I (London-New York, 1965), 15, 124–125. His other researches are listed in Proceedings of the Philosophical Society of Glasgow, 23 (1891–1892), 310.