74 



CHEMISTRY FOR AGRICULTURAL STUDENTS 



and of the intermediate hydrate, and represent them by 

 formulae. 



The intermediate hydrate, which should have a composition represented 

 by the formula 2CaS04.H20, is "Plaster of Paris." This, when mixed 

 with a little water, combines and sets to a hard mass of gypsum. Plaster of 

 Paris is prepared by heating gypsum. If the temperature be raised too 

 high, the anhydrous sulphate, which combines with water very slowly, is 

 formed, and it becomes useless as a plaster. 



Observe that each of the six salts is distinguished by a 

 different crystalline form. Examine the crystals of Epsom salt 

 with a lens. Note that each crystal is a prism, having the form 

 A (Fig. 30) or the modification B. Recrystallise some of the 



/V 



A 



Fig. 30. 



alum by dissolving in hot water and allowing the solution to 

 cool. Note that each crystal is an octahedron having the form 

 A (Fig. 31), or, owing to undue development of opposite faces 



Fig. 31. 



of the crystal, the modification B. Select one of the most 

 perfect crystals, suspend it by means of a hair from a glass rod, 



