American Heritage Dictionary
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Century Dictionary
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GNU Webster's 1913
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WordNet
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Elsewhere on the web
The first limitation is the oxygen -- oxygen toxicity.— Richard Pyle dives the reef's Twilight Zone
If the pressure as opposed to the oxygen is the dimension of interest then 1.3 atmospheres at 21\% could have been compared with 27. 3\% at ambient pressure.— BioMed Central - Latest articles
This experiment not only proves that the vermilion colour of the blood proceeds from oxygen, but likewise seems to show, that oxygen is the cause of excitability A quantity of azotic gas, which had been exposed for some time to the contact of lime water, in order to separate any carbonic acid gas it might contain, was injected into the jugular vein of a dog.— Popular Lectures on Zoonomia Or The Laws of Animal Life, in Health and Disease

American Heritage Dictionary (1)
Century Dictionary (1)
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