322 GYPSUM. 



sorbing carbonic acid but slowly from tlie air, is the 

 cause of this (248). Limestones which contain much 

 alumina and oxide of iron do not burn into good 

 lime, because the lime and clay partly unite and form 

 a kind of slag. 



875. Chalk, shell-sand, and indeed shells them- 

 selves, which consist of carbonate of lime and a small 

 quantity of animal matter, are useful additions to all 

 soils which contain but little lime. Quick or caustic 

 lime is especially useful in soils rich in humus. Soils 

 of this kind generally contain a small quantity of 

 acid, which greatly interferes with their fertility ; 

 this acid is neutralized by the addition of lime. Both 

 lime, and likewise its carbonate, are valuable addi- 

 tions to soils containing sulphate of iron (289, 663). 

 That salt which is hurtful to vegetation, is decom- 

 posed by lime ; gypsum is formed, whilst the iron 

 remains as an oxide or carbonate. 



876. Sulphate of lime, or gypsum, is likewise a 

 very useful addition to all soils which do not natu- 

 rally contain it. It is partly useful as supplying lime 

 and sulphuric acid, and partly a^ serving to fix am- 

 monia from the air (794), and thus yielding plants 

 more of that substance than they could otherwise 

 obtain. Gypsum is sometimes employed after being 

 burnt, and sometimes unburnt ; the only use of the 

 burning is that it renders it very easily crushed, the 

 gypsum in fact falling to powder when burnt. This 

 operation is rather roasting than burning, because, 



