THE SULPHATES 



75 



then having filtered the solution of alum into a beaker, rest the 

 rod across the top of the beaker as figured, so that the crystal 



Fig. 32. 



remains suspended in the solution. Watch the gradual growth 

 of the octahedron. Observe that in the case of selenite (Fig. 



Fig. 33. 



33) the crystals are connected together in the form of "twin 

 crystals." 



It has been noticed already that whereas zinc only when 

 strongly heated attacks water with liberation of hydrogen and 

 formation of oxide of zinc, it readily attacks water acidified with 

 sulphuric acid, hydrogen being briskly evolved and a clear liquid 



