COMPOUNDS OF CARBON" WITH OXYGEN AND NITROGEN 883 



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 



CaC0 3 + 2HC1 = CaCl 2 + H 2 + C0 2 . 



The nature of the reaction in this case is the same as in the decom- 

 position of nitre by sulphuric acid ; only in the latter case a hydrate is 

 formed, and in the former an anhydride of the acid, because the 

 hydrate, carbonic acid, H 2 C0 3 , is unstable and as soon as it separates 

 decomposes into water and its own anhydride. It is evident from the 

 explanation of the cause of the action of sulphuric acid on nitre that 

 not every acid can be employed for obtaining carbonic anhydride ; 

 namely, those will not set it free which chemically are but slightly 

 energetic, or those which are insoluble in water, or are themselves as 

 volatile as carbonic anhydride. 3 But as many acids are soluble in 

 water and are less volatile than carbonic anhydride, the latter is 

 evolved by the action of most acids on its salts, and this reaction takes 

 place at ordinary temperatures. 4 



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) which surrounds the untouched calcium carbonate, and thus 

 prevents a further evolution of gas. But if porous limestone for instance, chalk be 

 treated with sulphuric acid diluted with an equal volume of water, the liquid 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 baused ; 

 for instance, washing-soda, Na 2 CO 3 , which is often chosen when it is required to produce 

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

 crystalline magnesium carbonate and similar salts are with difficulty decomposed by 

 hydrochloric and sulphuric acids. When for manufacturing purposes for instance, in 

 precipitating lime in sugar-works a large quantity of carbonic acid gas is required, it 

 is generally obtained by burning charcoal, and the products of combustion, rich in 

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

 anhydride is thus absorbed. Another- method is also practised, which consists in using 

 the carbonic anhydride separated during fermentation, or that evolved from limekilns. 

 During the fermentation of sweet-wort, grape-juice, and other similar saccharine solu- 

 tions, the glucose C 6 H 12 O 6 changed under the influence of the yeast organism, forming 

 alcohol (2C 2 H 6 O), and carbonic anhydride (2C02) which separates in the form of gas ; if 

 the fermentation proceeds in closed bottles sparkling wine is obtained. When carbonic 

 acid gas is prepared for saturating water and other beverages it is necessary to use it 

 in a pure, state. Whilst in the state in which it is evolved from ordinary limestones by 

 the aid of acids it contains, besides a certain quantity of acid, the organic matters of the 

 limestone ; in order to diminish the quantity of these substances the densest kinds of 

 dolomites are used, which contain less organic matter, and the gas formed is passed 

 through various washing apparatus, and then through a solution of potassium perman- 

 ganate, which absorbs organic matter and does not take up carbonic anhydride. 



3 Hypochlorous acid, HC1O, and its anhydride, C1 2 O, do not displace carbonic acid, and 

 hydrogen sulphide has the same relation to carbonic acid as nitric acid to hydrochloric 

 an excess of either one displaces the other. 



4 Thus, in preparing the ordinary effervescing powders, sodium bicarbonate (or acid 



