The efficacy for the euthanasia of day-old chicks of mixtures of carbon dioxide and air, or carbon dioxide and argon containing 1, 2 or 5 per cent residual oxygen, or argon containing 1 or 2 per cent residual oxygen was tested in three experiments. The time to the onset of unconsciousness of individual chicks, determined from the time to loss of posture, was similar during their exposure to 2 per cent oxygen in argon, 20, 30 or 40 per cent carbon dioxide in argon with 2 per cent residual oxygen, or 90 per cent carbon dioxide in air. The exposure of chicks in batches of 20 to a mixture of 20, 30 or 40 per cent carbon dioxide in argon resulted in the death of all the chicks within two minutes. However, a residual oxygen level of 5 per cent in these mixtures resulted in the survival of some chicks for longer than two minutes. With argon alone the level of residual oxygen was critical; less than 2 per cent was essential to achieve 100 per cent mortality within three minutes, and a rise from 2 to about 3 per cent resulted in up to 20 per cent of the chicks surviving for seven minutes.
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