382 



PRINCIPLES OF CHEMISTRY 



Carbonates with various other bases such as, for instance, magnesia, 

 ferrous oxide, zinc oxide, &c. are often found in nature. The shells of 

 molluscs also have the composition CaCO 3 , and many limestones were 

 exclusively formed from the shells of minute organisms. As carbonic 

 anhydride (together with water) is produced during the combustion of 

 all organic compounds in a stream of oxygen or by heating them with 

 substances which readily part with their oxygen for instance, with 

 copper oxide this method is employed for estimating the amount of 

 carbon in organic compounds, more especially as the C0 2 can be easily 

 collected and the amount of carbon calculated from its weight. For 

 this purpose a hard glass tube, closed at one end, is filled with a 

 mixture of the organic substance (about 0'2 gram) and copper oxide. 

 The open end of the tube is fitted with a cork and tube containing 



Fro. 62. Apparatus for the combustion of organic substances by igniting thm with 

 oxide of copper. 



calcium chloride for absorbing the water formed by the oxidation of 

 the substance. This tube is hermetically connected (by a caoutchouc 

 tube) with potash bulbs or other weighing apparatus (Chapter V.) con- 

 taining alkali destined to absorb the carbonic anhydride. The increase 

 in weight of this apparatus shows the amounts of carbonic anhydride 

 formed during the combustion of the given substance, and the quantity 

 of carbon may be determined from this, because three parts of carbon 

 give eleven parts of carbonic anhydride. 



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 hydrochloric acid, 

 HC1 because, in the first place, the substance formed, calcium 

 chloride, CaCl 2 , is soluble in water and does not hinder the further 



