"In the second place, as all the earths and stones called vitrifiable have, notwithstanding their impurity, more hardness and transparency than others, and are fitter to communicate these good qualities to glass, they are employed preferably to any other earths in the composition, of glass, or artificial crystal. These are the only reasons why this kind of earth has been called vitrifiable. But we ought not from thence to conclude, that the earthy substance [i.e., the earth element or principle] which almost entirely composes them is more fusible and more vitrifiable than other earths: on the contrary... vitrifiable earth, when very pure, is of all earths the least fusible, and the least vitrifiable."