Carbonic acid

Had she lived now, she would have talked of the last delightful lecture on gas, or the charming new treatise on carbonic acid; she would have studied German, and delighted in the society of "talented people."

[I]t sank down through the air and poured over the ground in a manner rather liquid than gaseous, abandoning the hills, and streaming into the valleys and ditches and water-courses even as I have heard the carbonic acid gas that pours from volcanic clefts is wont to do.