372 



PRINCIPLES OF CHEMISTRY 



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, 

 MgCO 3 (in dolomite), the separation is easily effected, particularly in the 

 presence of the vapours of water. The acid salts of carbonic acid (see 

 further on) readily give much carbonic anhydride when heated. 

 Carbonic anhydride, together with water, is produced during the com- 

 bustion of all organic compounds in a stream of oxygen or by heating 

 them with substances which readily part with oxygen to them for 

 instance, with copper oxide, potassium chlorate (readily causing an 

 explosion), 



FIG. 62. -Apparatus for the combustion of organic substances by igniting tiicrn with 

 oxide of copper. 



The method of estimating the amount of carbon in organic com- 

 pounds is founded on this property. For this purpose a glass tube, closed 

 at one end, is filled with a mixture of the organic substance (about 0*2 

 grams) and copper oxide. The open end of the tube is fitted with a 



5 The direct observations made (1876) by Messrs. Bogouski and Kayander lead to the 

 conclusion that the quantity of carbonic anhydride evolved by the action of acids on 

 marble (as homogeneous as possible) is directly proportional to the time 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 86'5 

 ( = HC1) grams, about 0'73 grams 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 same quantity of carbonic anhydride 

 will be evolved ; thus, if the litre contains 68 ( = HNO 3 ) grams of nitric acid, or 81 ( = HBr) 

 grams of hydrobromic acid, the quantity of carbonic anhydride evolved will still be 

 0'78 gram. 



