Co.Ml'nt'NDS OF CAR 150 N WITH OXYGK.V AND NJTRoi iKA 371 



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, but 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 almost all the known acids 

 are characterised by their great solubility 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 tempera- 

 tures, provided the acid be soluble in water and that the calcium salt 

 formed is also soluble in water. 4 



For the preparation of carbonic anhydride in laboratories marble is 

 generally used. It is placed in a Woulfe's bottle and treated with hydro- 

 chloric acid in an apparatus similar to the one used for the production 

 of hydrogen. The gas evolved carries away through the tube part of 

 the volatile hydrochloric acid, and it is therefore necessary to wash the 

 gas by passing it through another Woulfe's bottle containing water. If 



al)sorl)'tl. Another method is also practised, which consists in using the carbonic 

 anhydride separated during fermentation, or that evolved from lime-kilns. During the 

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

 glucose C 6 H 13 O 6 changes under the influence of the yeast organism, forming alcohol 

 (2C._,H 6 O) and carbonic anhydride (2CO. 2 ), 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 potas- 

 sium permanganate, which absorbs organic matter and does not take up carbonic 

 anhydride. 



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 carbonate of soda) is used, and mixed with powdered citric or tartaric acid. In a 

 dry state these powders do not evolve carbonic anhydride, but when mixed with water the 

 evolution takes place briskly, which is due to the substance passing into solution. The 

 -alts of carbonic acid may be recognised from the fact that they evolve carbonic acid 

 with a hissing noise when treated with acids. If vinegar, which contains acetic acid, be 

 pouivd upon limestone, marble, malachite (contains copper carbonate), etc., carbonic 

 anhydride is evolved with a hissing noise. It is necessary to remark that without the 

 presence of water neither hydrochloric acid, nor even sulphuric acid nor acetic acid, acts 

 on limestone. We shall refer to this hereafter. 



B B 2 



