iv CHALK, LIME, AND THE ALKALIS 61 



silver and gold the residue left on heating a salt often con- 

 sisted of the revivified metal. In cases such as these the 

 base of the salt could only be separated with the help of an 

 alkali. 



SUMMARY AND SUPPLEMENT 

 A. CHALK AND LIME 



Joseph Black (1728-1799), in his "Experiments upon Mag- 

 nesia Alba, Quick-lime, and some other Alcaline Substances," 

 read in June 1755, showed 



(r) That chalk loses in weight by about 44 per cent, when 

 burnt to lime. 



(2) That, as nothing but a trace of water could be condensed 

 by cooling the vapour (Margraaf), this loss in weight must be 

 due to the escape of gas, to which he gave the name fixed air. 



(3) That lime combines with water to form slaked lime, but 

 releases it when it re-combrnes with fixed air to form chalk. 



(4) That fixed air is present in small quantities in common 

 air and in air dissolved in water. 



(5) That fixed air is liberated from chalk by the action of 

 acids ; apart from this liberation of gas, the action of acids upon 

 chalk and lime is identical both qualitatively and quantitatively. 



These changes may be represented by the following equa- 

 tions : 



Burning of chalk, CaCO 3 -> CaO + CO 2 



chalk -> lime + fixed air 



(Calcium (Calcium (Carbon 

 carbonate.) oxide.) dioxide.) 



Slaking of lime, CaO-f OH 2 -> Ca(OH) 2 



lime + water -> slaked lime 



(Calcium hydroxide.) 



Action of fixed air on slaked lime 



Ca(OH) 2 + CO 2 -> CaCO 3 + OH 2 

 slaked lime + fixed air -> chalk 4- water 



Action of acids on chalk, slaked lime, and quicklime 



(CaCO 3 + H 9 SO 4 - CaSO 4 + 

 i. Oil of mtriol Ca(OH) 2 + H 2 SO 4 -> CaSO 4 + 2H 2 

 +H 2 SO 4 -> CaSO 4 + H 2 O 



