xxr CARBON AND SOMK OF ITS COMPOUNDS 287 



the lime is raked out. If a piece of freshly formed lime can be 

 obtained it should be examined, 



EXPT. 293. To some fresh lime add some cold water, and 

 observe that the mass gets very hot and swells up, still re- 

 maining a solid. 



This lime is known as quick lime, and by this addition of 

 water becomes in some way changed, since the further addition 

 of water causes no further evolution of heat. The altered lime 

 is known as slaked lime. 



To find out the change undergone by the chalk in limestone 

 a small portion should be heated, and the gain or loss of weight 

 recorded. This can be done if a furnace a muffle furnace is 

 very convenient is available. 



EXPT. 294. Weigh a crucible and in it place some 

 powdered chalk. l Heat it in a muffle furnace at a bright red 

 heat for a few hours, allow it to cool, and again weigh. Cal- 

 culate the loss or gain for 100 grams of chalk. 



It will be found that the chalk loses about 44 per cent, of its 

 weight, and it is thus proved that the lime consists of chalk 

 from which some constituent has been expelled by heat. 



EXPT. 295. To a little chalk in a test-tube add some 

 dilute hydrochloric acid. Note brisk effervescence occurs. 

 Fit a delivery tube to the test-tube and pass the gas evolved 

 through lime-water, and observe the formation of a white 

 precipitate resembling that produced by carbon dioxide. To 

 some of the precipitate add a little dilute acid, and observe it 

 becomes clear, with slight effervescence. Evaporate the solu- 

 tion left in the test-tube and see that, as in the case of the 

 action between lime and hydrochloric acid, calcium chloride 

 is left. 



From chalk then, by the action of hydrochloric acid, we 

 obtain carbon dioxide and the product which results from the 

 interaction of the same acid with lime. It appears therefore 

 probable that the chalk consists of the carbon dioxide and the 

 lime. The next experiment will prove whether or not this is 

 the case. 



1 It is worth wliile to point out that the prepared substance used for black-board 

 writing is often not chalk. 



