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Darwin Correspondence Project

From August Wilhelm von Hofmann to Edward Cresy1   13 October 1860

R. College of Chemistry

Oct. 13. 1860.

My dear Sir

In reply to your note I hasten to state that I have not myself made many special experiments on the delicacy of tests.2 I must therefore quote to you such results as are known to me to have been obtained by others, stating at the same time the authority.

1 part of arsenic is detected in 64.800 parts of solution by Marsh’s test. (Taylor)3 1 part of lead in 158.400 parts of solutions by Sulphide of ammonium. (Taylor)4 1 part of iodine in 4.000.000 parts of solution by starch (Price)5

Hoping that these illustrations will suit you and Mr. Darwin, whose experiments are really of very great interest. I remain | My dear Sir | Yours very sincerely | A W. Hofmann

E. Creasy Esq.


The letter was forwarded to CD by Edward Cresy. The renowned German organic chemist August von Hofmann was director of the Royal College of Chemistry, part of the Government School of Mines. The school at one time came under the administration of the Metropolitan Board of Works, of which Cresy was principal assistant clerk. Cresy had visited CD in September 1860 (see ‘Journal’; Appendix II), at which time CD was engaged in experiments to determine the sensitivity of Drosera leaves to certain chemical substances.
Hofmann’s great reputation in organic chemistry primarily rested on his work on the nitrogenous bases. The laboratory of the Royal College of Chemistry was well known for its experimental accuracy.
James Marsh, Michael Faraday’s assistant at the Royal Institution of Great Britain and a practical chemist at the Royal Military Academy, London, invented the test for arsenic that bears his name. The test involved passing hydrogen gas through arsenical compounds. In recognition of this valuable toxocological achievement, Marsh was awarded the gold medal of the Society of Arts in 1836 (DNB). The reference is to Alfred Swaine Taylor’s authoritative textbook on poisons, in which the procedure for Marsh’s test is described (Taylor 1848, pp. 345–6). Taylor remarked that Marsh’s test was ‘undoubtedly one of great delicacy’ (Taylor 1848, pp. 350–1). See also Coley 1991.
In Taylor 1848, pp. 153–4, the method of detecting lead in solution with ‘hydrosulpuret of ammonia’ is described. Taylor noted that treating lead acetate solution in this way produced a deep black or brown precipitate ‘even when less than the 100,000th part of the salt is dissolved’ (ibid., p. 436).
Astley Paston Price was Hofmann’s assistant at the Royal College of Chemistry and a consulting chemist from 1857. There is no record of his particular test for iodine. Iodine is readily detected by the characteristic blue colour that it immediately produces upon contact with starch paste. In Taylor 1848, p. 305, it is stated that this test is a very delicate one.


DNB: Dictionary of national biography. Edited by Leslie Stephen and Sidney Lee. 63 vols. and 2 supplements (6 vols.). London: Smith, Elder & Co. 1885–1912. Dictionary of national biography 1912–90. Edited by H. W. C. Davis et al. 9 vols. London: Oxford University Press. 1927–96.

Taylor, Alfred Swaine. 1848. On poisons in relation to medical jurisprudence and medicine. London.


Has not himself experimented with delicacy of tests but sends several illustrations of what other authorities have done. Reference to James Marsh’s test for arsenic and that of Ashley Paston Price for iodine.

Letter details

Letter no.
August Wilhelm von Hofmann
Edward Cresy, Jr
Source of text
DAR 58.1: 4
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 2947B,” accessed on 13 August 2020,

Also published in The Correspondence of Charles Darwin, vol. 8