The Story-Book of the Fields 



granulated ; sometimes delicately fibrous, 

 with undulating reflections ; or again trans- 

 parent as glass and separating into very thin 

 sheets, exhibiting here and there the splendid 

 colours of the rainbow. This beautiful trans- 

 parent gypsum was used by the ancients for 

 window panes. 



The impure gypsum, or shapeless stone, is 

 used for common plaster, while the purer 

 kinds provide the finer sort that is intended 

 for moulding. The plaster stone is very 

 common, forming whole hills or mountains in 

 certain places. In order to prepare it for use 

 a moderate amount of heat is required. For 

 this purpose a number of small arches are 

 built up with lumps of gypsum and other 

 smaller pieces are piled over these. The whole 

 mass is baked by burning wood and brush- 

 wood underneath. 



Gypsum consists of lime similar to that 

 contained in chalk, but it is combined with 

 sulphuric acid, which cannot be expelled by 

 heat. It also contains water which forms 

 one-fifth of the whole weight of the stone. It 

 is only the water that is removed by the heat, 

 and as soon as it is rid of that, the gypsum 

 becomes plaster. 



But this has a great tendency to absorb 



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