384 PRINCIPLES OP CHEMISTRY 



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 

 it be necessary to obtain dry carbonic anhydride, it must be passed 

 through chloride of calcium. 5 



Carbonic anhydride may also be prepared by heating many of the 

 salts of carbonic acid ; for instance, by heating magnesium carbonate, 

 MgC0 3 (e.g., in the form of dolomite), the separation is easily effected, 

 particularly in the presence of the vapours of water. The acid salts 

 of carbonic acid (for instance, NaHC0 3 , see further on) readily and 

 abundantly give carbonic anhydride when heated. 



Carbonic anhydride is colourless, has a slight smell and a faint 

 acid taste ; its density in a gaseous state is twenty- two times as great 

 as that of hydrogen, because its molecular weight is forty - four. 6 



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

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

 evolution takes place briskly, which it* due to the substances passing into solution. The 

 salts 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 

 poured upon limestone, marble, malachite (containing copper carbonate), &c., carbonic 

 anhydride is evolved with a hissing noise. It is noteworthy that x neither hydrochlorici 

 acid, nor even sulphuric acid nor acetic acid, acts on limestone except in presence of 

 water. We shall refer to this later on. 



5 The direct observations made (1876) by Me.ssrs. Bogonski and Kayander kad to the' 

 conclusion that the quantity of carbonic anhydride evolved by the action of acida on , 

 marble (as homogeneous as possible) is directly proportional to the tim6 of action, the 

 extent of surface, and the degree of concentration of the acid, and inversely proportional 

 to the molecular weight of the acid. If the surface of a piece of Carrara marble be equal 

 to one decimetre, the time of action one minute, and one cubic decimetre or litre contains 

 one gram of hydrochloric acid, then about 0'02 gram of carbonic anhydride will be 

 evolved. If the litre contains n grams of hydrochloric acid, then by experiment the 

 amount will be n x 0'02 of carbonic anhydride. Therefore, if the litre contains S6;& 

 ( = HC1) grams, about 0'73 gram of carbonic anhydride (about half a litre) would be, 

 evolved per minute. If nitric acid or hydrobromic acid be used instead of hydrochloric, 

 then, with a combining proportion of the acid, the aame quantity of carbonic anhydride 

 will be evolved; thus, if the litre contains 63 ( = HNO 3 ) grams of nitric acid, or 81 

 < = HBr) grams of hydrobromic acid, the quantity of carbonic anhydride evolved will still, 

 "be 0'73 gram. Spring, in 1890, made a series of similar determinations. 



6 As carbonic anhydride is one and a half times heavier than air, it diffuses with 

 difficulty, and therefore does not easily mix with air, but sinks in it. This may be shown 

 in various ways; for instance, the gas may be carefully poured from one vessel into another 

 containing air. If a lighted taper be plunged into the vessel containing carbonic anhy* 

 dride it is extinguished, and then, after pouring the gas into the other cylinder, it will 

 burn in the former and be extinguished in the latter. If a certain quantity of carbonic 

 anhydride be poured into a vessel containing' air, and soap-bubbles be introduced, they. 

 will only sink as far as the stratum where the atmosphere of carbonic anhydride com* 

 anences, as this latter is heavier than the soap-bubbles filled with air. Naturally, after a 



