370 PRINCIPLES OF CHEMISTRY 



dride of the air constitutes the fundamental food of plants (Chaps. III., 

 V., and VIII.). Carbonic anhydride, in a state of combination with 

 most varied substances, is perhaps even more widely diffused in nature 

 than in a free state. Some of these substances are very stable, 

 and form a large portion of the earth's crust. In this state carbonic 

 anhydride enters into the composition of limestones. Limestones, 

 calcium carbonate CaCO^, were formed as precipitates in the seas 

 existing previously on the earth ; this is proved by their stratified 

 structure and the number of remains of sea animals which they 

 frequently contain. It may be concluded, judging from the enormous 

 quantity of these limestones, that the amount of carbon in the form of 

 carbonic anhydride in the atmosphere was in former periods much 

 greater than at present. Chalk, lithographic stone, limestone, marls (a 

 mixture of limestone and clay), and many other rocks are examples of 

 such sedimentary formations. Carbonates with various other bases 

 such as, for instance, magnesia, ferrous oxide, zinc oxide, &c. are 

 often found in nature. The shells of molluscs have also the composi- 

 tion CaCO 3 , and many limestones were exclusively formed from the 

 shells of minute organisms. 



For the preparation of carbonic anhydride in laboratories and 

 often in manufactories, various kinds of calcium carbonate are used, 

 being treated with some acid ; it is, however, most usual to employ 

 the so-called muriatic acid that is, an aqueous solution of hydro- 

 chloric acid, HC1 because, in the first place, the substance formed, 

 calcium chloride, OaCl. 2 , is soluble in water and does not hinder the 

 further action of the acid on the calcium carbonate, and secondly 

 because, as we shall see further on, muriatic acid is a common product 

 of chemical works and one of the cheapest. For calcium carbonate, 

 either limestone, chalk, or marble is used.' 2 



CaCO 3 + 2HCl=CaCl 2 + H 2 O + CO 2 . 



2 Other acids may be used instead of hydrochloric ; for instance, acetic, or even 

 sulphuric, although this latter is not suitable, because it forms as a product insoluble 

 calcium sulphate (gypsum) surrounding the untouched calcium carbonate, and thus pre- 

 venting a further evolution of gas. But if porous limestone for instance, chalk bo 

 treated with sulphuric acid diluted with an equal volume of water, the latter is absorbed, 

 and, acting on the mass of the salt, the evolution of carbonic anhydride continues evenly 

 for a long time. Instead of calcium carbonate, other carbonates may of course be used ; 

 for instance, washing-soda NaoCO 3 , which is often chosen when it is required to produce a 

 rapid stream of carbonic anhydride (for example, for liquefying it). But natural crystal- 

 line magnesium carbonate and similar salts are with difficulty decomposed by hydro- 

 chloric and sulphuric acids. When for manufacturing purposes for instance, in pre- 

 cipitating lime in sugar-works a large quantity of carbonic acid gas is required; 

 generally charcoal is burnt, and the products of combustion, rich in carbonic anhydride, 

 are pumped into the liquid containing the lime, and the carbonic anhydride is thus 



