To help in elucidating this matter I have made some experiments as to the detection of the odour of prussic acid. An acid of 2 per cent. strength (B.P.) was used.
1. From a bottle of Guinness’s stout, freshly opened, 3 samples of 1 fluid oz. each were measured. To the first 1 drop of the acid was added, to the second 2 drops, the third being left untouched. This was done out of my sight in another room. They were then privately marked by an assistant, and brought in; when myself and two others, one of them entirely inexperienced, independently and at once classified them without hesitation correctly as to the relative amounts of prussic acid. The odour was so distinct as to produce, when inhaled, a feeling of oppression, and to quite overpower the odour of the beer.
| 1 drop in 1 fl. oz. | = | 0·23 per cent. of the dilute (B.P.) acid. |
| ” | = | ·0046 per cent. of real anhydrous HCN. |
About 1/30 of a poisonous dose. Hence if a poisonous dose were put into a pint and a half of stout, the odour would be distinct.
2. One drop of the dilute (2 per cent.) acid was added to 6 oz. stout: there resulted a slight but distinct odour of prussic acid. Hence a poisonous dose in nine pints would be smelt. Covered by a watch-glass, with a drop of yellow ammonium sulphide on it, it was warmed; the drop, on evaporation, gave a distinct red sulphocyanide reaction with ferric chloride. Exposed to the air for twenty-four hours, all the above samples had lost their odour, and failed to give the sulphocyanide reaction.
3. Two samples of urine, measuring ½ pint each, were treated respectively with 1 and 2 drops of B.P. acid (strength in this case 1·18 per cent. HCN), and a third ½ pint left untouched, the same precautions being used as with the above beers. Three independent witnesses again classified them without difficulty as to relative amounts of the poison. This is 2 drops of an exceptionally weak acid to the pint.
4. The contents of a human stomach, very fetid from decomposition, were divided into two portions of about 2 oz. each: one was left untouched; to the other 1 grain of mercuric cyanide was added, and then about 5 drops of hydrochloric acid, and a little zinc dust. The whole was well stirred, and shut up close. Next day the odour of HCN was very prominent in the one to which the cyanide had been added, in spite of the strong original smell of both.
5. I cannot agree with Taylor that either peppermint or tobacco mask the odour appreciably.
The odour of nitrobenzol, being similar to that of bitter almonds, might lead to a suspicion of prussic acid without due caution (Woodman and Tidy).
In putrefying, organic matters often develope ammonium sulphide, becoming alkaline. The ammonium sulphide would combine with the HCN to form sulphocyanide of ammonium, which is inodorous, but, by distillation with acids, gives HCN. Sulphocyanide, however, could not be produced unless the original matters were alkaline. In Tawell’s, and most other trials, the stomach contents were acid, as they always are naturally from the gastric juice.