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."
1 Greek St
30 Oct 60.
My dear Sir,
From Taylor on Poisons, the book D
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
``This test (sulphuretted hydrogen) is extremely delicate in its
reaction.— It begins to give a yellow tinge when the liquid
``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; 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
''Delicacy of Reinsch's process— This test
failed to detect
``Corrosive sublimate— The protochloride of tin added to
Prussic acid— ``A standard solution was made in which each grain of liquid
- f1 2968.f1Taylor 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 , CD had inquired about the units of measurement used by Hofmann in the information he gave about the sensitivity of certain chemical tests.
- f2 2968.f2See letter to Edward Cresy, 2 November . From the notes CD made on his experiments in Eastbourne (DAR 54 and 60.1), it is evident that he was using a microscope.
- f3 2968.f3CD remained in Eastbourne until 10 November (`Journal'; Appendix II).
- f4 2968.f4The enclosure is in DAR 58.2: 49--52. For Alfred Swaine Taylor's work on the detection of poisons, see Coley 1991.
- f5 2968.f5Taylor 1848, p. 305.
- f6 2968.f6Thomas Stewart Traill was professor of medical jurisprudence at Edinburgh University.
- f7 2968.f7Taylor 1848, p. 339.
- f8 2968.f8Taylor 1848, p. 341.
- f9 2968.f9For James Marsh's test for arsenic, see the letter from A. W. von Hofmann to Edward Cresy, 13 October 1860, n. 3.
- f10 2968.f10Taylor 1848, p. 351.
- f11 2968.f11Edgar Hugo Emil Reinsch was a teacher of chemistry in Zweibr¨ucken, 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.
- f12 2968.f12Taylor 1848, p. 354.
- f13 2968.f13Taylor 1848, p. 409. Corrosive sublimate is mercuric chloride.
- f14 2968.f14Taylor 1848, p. 685.