mephitic acid

— The fact is, that fixed air is so rare a vapour and the Mephitic Acid, as I shall already venture to call it, is so greatly diluted in water, which is even saturated with it, that many of the blue juices relist its action upon them; while others, more sensible tests of acidity (such as infusions of Litmus, Cyanus, or Corn-flower, and a few others) readily announce its acid quality.