14 POISONS : THEIR EFFECTS AND DETECTION. [§ 10 . 
shrewd observations, bearing on toxicology, in his work on The Useful¬ 
ness of Natural Philosophy , etc. (Oxford, 1654). Nicolas L’Emery also 
wrote a Cours de Chimie —quite an epitome of the chemical science of 
the time. 1 
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, 2 without, however, noting 
its poisonous properties ; the same chemist separated oxalic acid from 
sorrel, 3 and made the important discovery that arsenic united with 
hydrogen, forming a foetid gas, and, moreover, that this gas could be 
decomposed by heat. 4 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 :— 
§ 10. Proust 5 observed that a very foetid 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. 6 Stromeyer, Gay- 
Lussac, Thenard, 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. 7 He constructed 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, etc. 
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 
1 Cours de Chimie, contenant la maniere de faire les operations qui sont en usage, 
dans la Medecine. Paris, 1675 
2 Opuscula Chemica, vol. ii. pp. 148-174. 
3 De Terra Rhubarhi et Acido Acetosellce. 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 
bodios ; but Scheele recognised its identity with the acid contained in sorrel. 
4 Memoires de Scheele, t. i., 1775. 
5 Proust, Annales de Chimie, t. xxviii., 1798. 
0 Nicholson's Journal, vol. vi. 
7 “ Description of a New Process of separating Small Quantities of Arsenic from 
Substances with which it is mixed,” Ed. New Phil. Journal, 1836. 
