In the eighteenth century still further advances were made. Richard Mead published his ingenious _Mechanical Theory of Poisons_. Great chemists arose--Stahl, Marggraf, Brandt, Bergmann, Scheele, Berthollet, Priestley, and lastly, Lavoisier--and chemistry, as a science, was born. Of the chemists quoted, Scheele, in relation to toxicology, stands chief. It was Scheele who discovered prussic acid,[20] without, however, noting its poisonous properties; the same chemist separated oxalic acid from sorrel,[21] and made the important discovery that arsenic united with hydrogen, forming a fœtid gas, and, moreover, that this gas could be decomposed by heat.[22] From this observation, a delicate test for arsenic was afterwards elaborated, which for the first time rendered the most tasteless and easily administered poison in the whole world at once the easiest of detection. The further history of what is now called “Marsh’s Test” is as follows:-- [20] _Opuscula Chemica_, vol. ii. pp. 148-174. [21] _De Terra Rhubarbi et Acido Acetosellæ._ _Nova Acta Acad. Veg. Sued. Anni_, 1784. _Opuscula Chemica_, vol. ii. pp. 187-195. Bergmann first described oxalic acid as obtained by the oxidation of saccharine bodies; but Scheele recognised its identity with the acid contained in sorrel. [22] _Mémoires de Scheele_, t. i., 1775. § 10. Proust[23] observed that a very fœtid hydrogen gas was disengaged when arsenical tin was dissolved in hydrochloric acid, and that arsenic was deposited from the inflamed gas on cold surfaces which the flame touched. Trommsdorff next announced, in 1803, that when arsenical zinc was introduced into an ordinary flask with water and sulphuric acid, an arsenical hydrogen was disengaged; and if the tube was sufficiently long, arsenic was deposited on its walls.[24] Stromeyer, Gay-Lussac, Thénard, Gehlen, and Davy later studied this gas, and Serullas in 1821 proposed this reaction as a toxicological test. Lastly, in 1836, Marsh published his Memoir.[25] He elaborated a special apparatus of great simplicity, developed hydrogen by means of zinc and sulphuric acid, inflamed the issuing gas, and obtained any arsenic present as a metal, which could be afterwards converted into arsenious acid, &c. [23] Proust, _Annales de Chimie_, t. xxviii., 1798. [24] _Nicholson’s Journal_, vol. vi. [25] “Description of a New Process of Separating Small Quantities of Arsenic from Substances with which it is mixed.” _Ed. New. Phil. Journal_, 1836. This brief history of the so-called “Marsh’s Test” amply shows that Marsh was not the discoverer of the test. Like many other useful processes, it seems to have been evolved by a combination of many minds. It may, however, be truly said that Marsh was the first who perfected the test and brought it prominently forward.