Limestone is also carbonate of lime, containing about 44 per cent, of its weight of carbonic acid, and is found in many places and in various forms and colors. Most limestones are neptunic formations and enclose more or less organic matters, animal and vegetable parts, sulphur, bitumen, carbureted hydrogen, etc.

These impurities make limestone absolutely unfit for generating gas for the mineral-water trade. There are, however, some pure or nearly pure and even purified grades (purified by slightly heating it to destroy organic and bituminous matters - but the limestone loses a small amount of its carbonic acid by this process) in the market; however, great care has to be exercised in choosing the proper kind, and carbonators ought to examine the quality of gas in regard to its purity before deciding definitely. In reference to this examination see further on in this Chapter.

As far as known, limestone is scarcely used in this country; however, some manufacturers may have reasons to use or try it, reasons which may lie in local circumstances.

Limestone for use in the mineral-water factory is ground and can be decomposed either by sulphuric or muriatic acid, the former being preferable. When muriatic acid is used, it is better to use the limestone in coarse fragments, else the gas would be too suddenly liberated and cause violent foaming. The residue is the same as by whiting.