1356. The existence of carbonic acid gas in the atmosphere is proved by the following process: if a clear solution of lime water be exposed to the air, a pellicle will speedily form upon it, and a solid matter will gradually fall to the bottom of the water, and in a certain time the water will become tasteless: this is owing to the combination of the lime which was dissolved in the water with carbonic acid gas, which existed in the atmosphere, as may be proved by collecting the film and the solid matter, and igniting them strongly in a little tube of platina or iron; they will give out carbonic acid gas, and will become quicklime, which, added to the same water, will again bring it to the state of lime-water.
