Tuscan order A plain astragal or taenia ringed the column beneath its plain cap. Palladio agreed in essence with Serlio: "The Tuscan, being rough, is rarely used above ground except in one-storey buildings like villa barns or in huge structures like Amphitheatres and the like which, having many orders, can take this one in place of the Doric, under the Ionic." Unlike the other authors Palladio found Roman precedents, of which he named the arena of Verona and the Pula Arena, both of which, James Ackerman points out, are arcuated buildings that did not present columns and entablatures. A striking feature is
Ionic radius radii. The relation between soft-sphere ionic radii, formula_2 and formula_3, and formula_1, is given by formula_5, where formula_6 is an exponent that varies with the type of crystal structure. In the hard-sphere model, formula_6 would be 1, giving formula_8. In the soft-sphere model, formula_6 has a value between 1 and 2. For example, for crystals of group 1 halides with the sodium chloride structure, a value of 1.6667 gives good agreement with experiment. Some soft-sphere ionic radii are in the table. These radii are larger than the crystal radii given above (Li, 90 pm; Cl, 167 pm). Inter-ionic separations calculated