Taking magnesia alba, which he distinguished from limestone with which it had previously been confused, he showed that on being heated it lost weight owing to the escape of this fixed air (named carbonic acid by Lavoisier in 1781), and that the weight was regained when the calcined product was made to reabsorb the fixed air with which it had parted.
He determined the specific gravity of these gases with reference to common air, investigated the extent to which they are absorbed by various liquids, and noted that common air containing one part in nine by volume of fixed air is no longer able to support combustion, and that the air produced by fermentation and putrefaction has properties identical with those of fixed air obtained from marble.
Thus oxygen gas, at the end of the 18th century, was known as dephlogisticated air, nitrogen or azote as phlogisticated air, hydrogen as inflammable air, carbonic acid gas as fixed air.
Aquarius, a fixed air sign is on my 7th cusp, ruled by Saturn in Leo in 12.
