The Persistence of Hydrogen Cyanide in Rabbit Burrows

Abstract

Experiments were carried out in an artificial rabbit burrow to test the persistence of concentrations of hydrogen cyanide. The gas was evolved from two kinds of trade powders (Cymag and Cyanogas), placed in a heap just inside the mouth of the burrow, which was then sealed in with earth. Samples of the mixture inside the burrow were withdrawn from various points along its length at stated intervals, and the concentrations of hydrogen cyanide determined by a rapid and easy method, which is described, and plotted against time.Data on the toxicity of hydrogen cyanide to rabbits are considered and the following critical points determined: 1 part in 1250 by volume (1 mg./l.) kills 50 % of rabbits in 1 min.; 1 part in 5400 (0.22 mg./l.) kills in 18 min.; and 1 part in 7500 (0.16 mg./l.) is at the threshold of toxicity. Collapse, which is probably the most important point from a practical point of view, occurs in about a third of these times.The persistence of HCN concentration resulting from the use of half an ounce (14.15 g.) of either Cymag or Cyanogas cannot be regarded as sufficient to guarantee good results.With a dose of 1 oz. (28.3 g.) in summer a concentration of 1 part in 1250 is maintained for 34 hr. (Cyanogas) and for 48 hr. (Cymag), and a concentration of 1 part in 5400 for about 80 hr. by both materials. This means that a concentration which will kill rabbits in 1 min. can be maintained over the second night after gassing by either substance, but Cyanogas must be applied late in the day.Under winter conditions a concentration of 1 part in 1250 is only reached by Cymag for a short period between 16 and 25 hr. after the start of the experiments; by Cyanogas not at all. On the other hand, a concentration of moderate toxicity (1 part in 5400) is maintained for about 120 hr. Probably a dose somewhat greater than 1 oz. would be an advantage in the winter.In summer the initial rise in concentration is immediate: in winter there is a delay of some 10–15 hr. before a good concentration is built up. Under these conditions it will be better to gas rabbit warrens early in the day, or alternatively to scatter the powder slightly.In general, the characteristic differences shown between Cymag and Cyanogas are due to the buffering effect produced on the evolution of hydrogen cyanide from Cymag by the increasing concentration in the surrounding air. Thus the curve has a typical flat‐topped form, contrasting with the very high peak, quickly rising and falling, produced by Cyanogas.Diffusion of the hydrogen cyanide down the burrow away from the source is very small in the summer (a sampling point 40 in. away from the source was consistently negative throughout the experiments), and only slightly more in the winter.