292 ELEMENTARY GENERAL SCIENCE CHAP. 



water. Temporary hardness is in fact generally due to chalk, 

 but another closely allied compound, magnesium carbonate, 

 gives similar results. We may say, however, that it is due to 

 the carbonates of calcium or magnesium dissolved in water con- 

 taining carbon dioxide ; and the white solid obtained in Expt. 250 

 was probably one of these compounds. 



EXPT. 300. Shake a solution of calcium chloride with 

 soap solution. Observe that this is also very hard, but that 

 the hardness is not removed by boiling. The water is per- 

 manently hard. 



Permanent hardness is also generally due to a calcium 

 (or magnesium salt) soluble in water itself, and hence not pre- 

 cipitated by boiling ; calcium sulphate or magnesium sulphate 

 is the salt most commonly found in permanently hard water. 

 In either case, i.e., temporary or permanent hardness, the 

 calcium (or magnesium) unites with a portion of the soap and 

 hence destroys it, forming a precipitate which renders the water 

 turbid. Until sufficient soap is used to unite with the whole 

 of the calcium present no lather is obtainable. 



Lower Oxide of Carbon. If a coke or charcoal fire be ob- 

 served, flames of a peculiar blue colour are usually seen playing 

 over the top. As, however, the only substances concerned in 

 the combustion are carbon and oxygen, and since carbon itself 

 does not burn with a blue flame, while carbon dioxide is quite 

 incombustible, it appears probable that there must be some 

 other compound of carbon and oxygen which is produced and 

 which burns with these blue flames. Further, it seems probable 

 that the oxide is a lower oxide, i.e., contains less oxygen, as if 

 a compound with more oxygen existed we should expect the 

 carbon dioxide to burn and form this oxide. We may there- j 

 fore first endeavour to obtain this oxide by abstracting some of ( 

 the ox3'geii from carbon dioxide. 



EXPT. 301. In a hard glass tube AB, such as was used 

 in Expt. 234, place some iron filings. Pass over these filings 

 a current of carbon dioxide, dried in the usual manner. Heat 

 the filings, and let the end B of the tube dip under a strong 

 solution of caustic potash, which absorbs any unchanged 

 carbon dioxide, a tube containing the same liquid being inverted 

 over the end (Fig. 138. ) Observe that the bubbles of gas which 

 come from the end of the tube are almost completely absorbed, 



