This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
"(1). Quieting of the sensitive nerves of the stomach; (2). The stimulation of the secretions and the peristaltic action of the stomach; (3). The stimulation of the action of the bowels; (4). Increased secretion of the kidneys. In addition to these, he alludes to the importance which free carbonic acid possesses in the solution, and in the holding in solution, of the bicarbonates contained in mineral waters, especially the bicarbonates of soda and iron".
The imagination of many people that carbonic acid gas when breathed has a poisonous effect is erroneous. There are two compounds of carbon and oxygen - the oxide of carbon, and the carbon dioxide or carbonic acid. Both are unfit to breathe and the former is poisonous. Many of the deaths which are attributed to carbonic acid gas are really produced by the oxide of carbon. The oxide has neither odor, color nor taste, and being lighter than air it fills the upper portion of a room long before the carbonic acid, which spreads gradually over the floor. The former, moreover, produces injurious effects when mixed with air even in so small a proportion as one-half per cent., and four or five per cent, of it is fatal, whereas it requires about thirty per cent, of carbonic acid gas to produce death. The difference between the action of the two gases is that the oxide acts directly as a poison, whereas the dioxide (carbonic acid) has a purely negative action. As the latter is heavier than air, it fills the lungs and excludes the air from them, thus causing asphyxiation exactly similar to that produced by drowning. As the oxide of carbon cannot be produced by the action of acid on the carbonates, there need be no fear of contaminating the beverages with it.