2. If a small amount of CaO is added to an otherwise mostly silica glass, what will happen to the (SiO4) tetrahedra in close proximity to calcium cations? 3. A rod-shaped MgO specimen with round cross section is tested with the three-point bend method. What is the calculated minimum radius without fracture if the applied load is 5560 N, the flexural strength is 105 MPa, and the separation between the two support pins is 45 mm? 4. In the appendices, the modulus of elasticity for silicon nitride is 304 GPa (that of steel is 207 GPa). How much porosity could be present in the round bar of silicon nitride such that it equals the reported value for steel?