The symptoms and post-mortem appearances of poisoning by Cyanide of Potassium are the same as those of prussic acid, except that:—
1. Convulsions are more common.
2. Owing to the irritant action of the alkali, the stomach is reddened.
3. The contents are alkaline.
The fatal dose is less than five grains, but Taylor mentions a case of recovery after nearly one ounce of the commercial cyanide, which may, however, have contained much carbonate.
Hydrocyanic acid is not, in the strict sense, a cumulative poison; “but doses that exceed the proper medicinal limit may happen to prove fatal though similar previous ones have appeared to be harmless, in consequence of a change in the body itself.” (Guy and Ferrier’s Forensic Medicine, 1881, p. 606.)
In the trial of George Ball for poisoning his mother with prussic acid, at Lewes, July, 1860 (previously reported), the question arose as to the difference between minims and drops. A minim of water is supposed to weigh a grain: if the fluid is heavier than water, it weighs more than a grain; if lighter, it weighs less. But a drop is quite an indefinite quantity: it is affected, not only by the specific gravity, but by the cohesion of the fluid, by the shape and size of the vessel, the manner of pouring, and the temperature. I have made some experiments which show the irregularity. (See also Woodman and Tidy’s Forensic Medicine, p. 456.)
| Capacity of Bottle. | Liquid. | No. of Drops. | Measured in Minims. | |
| (Stoppered) 6 fluid oz. | Water | 117 | 180 | |
| Do. (another observer) | Do. | 90 | 120 | |
| 1½ fluid oz. | Do. | 47 | 100 | |
| (Corked) 6 do. | Do. | 36 | 100 | |
| Same capacity, dropped with the cork | Do. | 37 to 41 | 100 | |
| (Stoppered) 6 fluid oz. | Rectified Spirit | 243 | 120 | |
Proving that while a drop may be estimated at about 1½ to 2 minims (a good deal more than the usual supposition, the two terms being often regarded as synonymous), yet the inconstancy is so great that it is absolutely imperative, in using powerful medicines, to prescribe exact measurement, and not such a precarious process as dropping.
As to the period after death during which HCN may be detected, Allen (Comm. Org. Anal.) asserts that its detection is rarely possible after more than twenty-four hours. This is astonishing, as Casper separated more than 18 milligrammes from a body eight days after death; Sokoloff detected it in hounds sixty days after; Dragendorff after four weeks in a dog, after eight or ten days in man. Reichardt (Arch. Pharm. 3, 19, 204) found it in a body two months after death—in the organs, but not in the urine. In the Tawell trial, also, the interval was considerable.