Koch, in his article on Disinfection and Antiseptics in the Mittheilungen aus dem K. Gesundheitsamte, vol. i., 1881, p. 264 (abstracted by Dr Whitelegge in the publications of the Kew Sydenham Society, vol. for 1886, “Microparasites in Disease”), gives the result of an elaborate series of experiments with mercuric chloride, and comes to the conclusion that a single application of a very dilute solution (1 to 1000, or even 1 to 5000) is sufficient to destroy the most resistant organism in a few minutes. He further states that, “with longer exposure, it only begins to be unreliable when diluted beyond 1 to 20,000.”