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Darwin Correspondence Project

From Edward Cresy   30 October 1860

1 Greek St

30 Oct 60.

My dear Sir,

From Taylor on Poisons, the book Dr Hofman refers to I have extracted the passages which I think will be useful or interesting to you—and which answer your question—1 the smallest portion of arsenic detectable seems 14000 grain and of iodine 11400 grain and 14400 grain of prussic acid, but as you will see much depends on the amount of dilution—   Your Drosera beats the chemists hollow, when you have time, I should like to know the details of your experiments    Are you quite sure of the amount of dilution and equally so of your negative experiments?— Have you eliminated all mechanical causes & effects due to breathing, perspiration soiling your glass rod in previous experiments—   I presume also your observation is made under the microscope.2 Still it is very curious—   I hope if I can do anything else for you, you will let me know—   I have addressed you at Down fearing you might have left Eastbourne—3 I only got Hofman’s answer yesterday—so he probably had not time till Sunday—

Yours very truly | E Cresy

Charles Darwin Esq—


diffused through 77,000 parts of water gave with starch a light pink colour and the colour became blue when the proportion amounted to 1470 grain in 28,000 parts of distilled water—5 Arsenic—“Dr Traill6 has lately asserted that 116000 part of a grain is precipitated by the silver test and that with 1/10,000 grain the precipitate is visible to the eye. I have found that 18000 grain dissolved in one drop of water gave a pale yellow film, but the result materially depended on the quantity of water present—   Thus 14000 grain of arsenic in ten drops of water was not perceptibly affected by the test but 12000 grain dissolved in four drops of water gave a decidedly yellow precipitate”—7

“This test (sulphuretted hydrogen) is extremely delicate in its reaction.—   It begins to give a yellow tinge when the liquid contains only 14000 grain of arsenious acid in ten drops of water the arsenic therefore forming about 1/40,000 part of the solution”—8

“There is no doubt that considerably less than the millionth part of a grain of arsenic may by Marsh’s test be rendered visible on a glass plate;9 it is possible to distinguish by the eye a piece of leaf gold which would weigh less than the ten-millionth part of a grain but the real question is whether the test will discover arsenic in a single drop of solution made by dissolving one grain of the poison in a million grains or sixteen gallons of water! If not the statement amounts to nothing for it is clear that if more than one drop of such an extremely diluted solution be taken the test is acting upon a larger quantity of arsenic than the above form of expression would indicate—   I have generally found that the fractional quantity stated to be detected referred rather to the degree of dilution than to the absolute quantity of poison present, whereas a test may fail to act as we have already seen either from the smallness of the quantity present or from the very large quantity of water in which it is diffused— The results of my own experiments are that where the arsenic is mixed with the acid liquid in a tube capable of holding two fluid ounces very faint and scarcely perceptible deposits begin to be formed on a glass plate with a quantity equal to the 12160 of a grain the diffusion here being equal to two million times the weight of the poison—   With 11080 grain in the same quantity of water the arsenic forming therefore one millionth part slight brown annular stains were procured. x x x x x x x x x— With 1720 grain the arsenic being in the proportion of about 1/800,000 of the liquid the stains were much more decided but quite imponderable— With 1100 grain in one fluid oz of water, 1/48,000 part, and 167 grain in 2 fluid ounces, 1/64,800, the deposits on glass were decided and characteristic—10

”Delicacy of Reinsch’s process—11 This test failed to detect 14000 grain in 30 drops of water the dilution being equal to 120,000 times the weight of arsenic— The deposit on copper commenced with a violet coloured film—when the quantity of arsenious acid was equal to 13000 grain in 30 drops of water or under a dilution of 90,000 times its own weight    It was also very decided with 12000 grain in the same quantity of water—x x x x x x— the copper was coated in a few seconds when boiled in a solution containing 14000 grain in ten drops of water though the test had failed to detect the same weight of arsenic in three times that quantity of water—12

“Corrosive sublimate—   The protochloride of tin added to 18640 grain of corrosive sublimate dissolved in one drop of water in a minute tube produced a dark grey discoloration from which in twenty four hours a black substance separated—   This however was in such small quantity that it was impossible to determine whether it did or did not contain any mercury”13

Prussic acid— “A standard solution was made in which each grain of liquid contained 14400 grain of anhydrous prussic acid— One grain of this very diluted liquid placed on a plate of glass—gave a decided milkiness with nitrate of silver but when the same quantity was mixed with sixty grains of water nitrate of silver had no perceptible effect shewing the powerful influence of dilution on the action of a test 14400 grain of prussic acid being in this instance diffused through 264000 parts of water— 11460 grain in sixty grains of water, dilution 87,600, gave with nitrate of silver a milkiness but no precipitate— With 1440 grain, dilution 26400, the milkiness produced by the test increased—& the white cyanide became flocculent—& with 1220 grain, dilution 13200, the test gave a decided precipitate”—14


Taylor 1848. Cresy refers to the remark made by August Wilhelm von Hofmann in the letter from A. W. von Hofmann to Edward Cresy, 27 October 1860, which Cresy forwarded to CD. See also the enclosure. In his letter to Edward Cresy, 14 October [1860], CD had inquired about the units of measurement used by Hofmann in the information he gave about the sensitivity of certain chemical tests.
See letter to Edward Cresy, 2 November [1860]. From the notes CD made on his experiments in Eastbourne (DAR 54 and 60.1), it is evident that he was using a microscope.
CD remained in Eastbourne until 10 November (‘Journal’; Appendix II).
The enclosure is in DAR 58.2: 49–52. For Alfred Swaine Taylor’s work on the detection of poisons, see Coley 1991.
Taylor 1848, p. 305.
Thomas Stewart Traill was professor of medical jurisprudence at Edinburgh University.
Taylor 1848, p. 339.
Taylor 1848, p. 341.
For James Marsh’s test for arsenic, see the letter from A. W. von Hofmann to Edward Cresy, 13 October 1860, n. 3.
Taylor 1848, p. 351.
Edgar Hugo Emil Reinsch was a teacher of chemistry in Zweibrücken, Germany. He outlined his test for the presence of arsenic in Reinsch 1841 and discussed further details of it in Reinsch 1842. The test involved boiling the substance with muriatic (hydrochloric) acid: if arsenic was present, a piece of copper wire would be discoloured by the mixture. See Taylor 1848, p. 352.
Taylor 1848, p. 354.
Taylor 1848, p. 409. Corrosive sublimate is mercuric chloride.
Taylor 1848, p. 685.


Reinsch, Edgar Hugo Emil. 1841. Ueber das Verhalten des metallischen Kupfers zu einigen Metalllösungen. Journal für Praktische Chemie 24: 244–50.

Reinsch, Edgar Hugo Emil. 1842. Ueber die Kupferarsenikprobe. Repertorium für die Pharmacie 2d ser. 27: 313–21.

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


Sends CD passages from A. S. Taylor’s book [On poisons in relation to medical jurisprudence and medicine, 2d ed. (1859)], citing smallest portions of poisons that are chemically detectable. "Drosera beats the chemists hollow."

Letter details

Letter no.
Edward Cresy, Jr
Charles Robert Darwin
Sent from
London, Greek St, 1
Source of text
DAR 58.1: 6, 58.2: 49–52
Physical description
ALS 2pp, 3 encls

Please cite as

Darwin Correspondence Project, “Letter no. 2968,” accessed on 30 March 2023,

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