PLASTER OF PARIS 57 



in appearance according to its state of purity. 

 Sometimes it is a shapeless rock, whitish and more 

 or less grained; sometimes a fine fibrous mass with 

 a silky luster ; or, again, a substance as transparent 

 as glass and splitting into very thin scales which 

 show, here and there, the superb colors of the rain- 

 bow. Struck by their beauty, workmen engaged in 

 quarrying gypsum have given the name of 'Jesus- 

 stone' to these brilliant laminae. Also, from their 

 brilliance and their cheapness, they are called 'don- 

 key's mirrors.' In ancient times these beautiful 

 sheets of transparent gypsum were used as window- 

 panes. 



"Impure gypsum, in the form of shapeless rock, 

 is used for ordinary plaster, while pure gypsum, 

 which comes in glass-like sheets or in blocks of a 

 silky appearance, is used for fine plaster, as in all 

 sorts of molding. The stone from which plaster is 

 obtained occurs in abundance in several departments 

 of France, where it forms hills and even whole moun- 

 tains, as for example in the departments of the Seine, 

 the Mouths of the Ehone, and Vaucluse. For con- 

 version into the usual plaster of Paris this stone 

 must be subjected to a moderate heat. To this end 

 it is the practice to build with gypsum blocks a row 

 of small vaults, and on these vaults to pile fragments 

 of smaller size. Then the firing is done by burning 

 fagots and brushwood under these vaults." 



"And is it carbonic acid gas this time, too, that 

 is driven out by the heat, as in the manufacture of 

 lime?" asked Jules, 



