Defects in crystals are responsible for the differences that exist between the properties of any given real crystal, or crystal aggregate, and those calculated for the corresponding "ideal" crystal; that is, calculated from the mathematical model of an ideally regular array of infinitely many atoms. With respect to a number of properties, such as density, elastic constants, specific heat, and melting point, the differences between real crystals and ideal crystals of the same type are minor. For example, if we calculate the temperature dependence of the specific heat of, say, rocksalt, by considering an ideal array of sodium and chlorine ions in a cubic arrangement, we find a satisfactory agreement between theory and experimental fact. However, besides such "structure-insensitive" properties, there are many "structure-sensitive" properties which reveal enormous differences between ideal and real crystals. In particular, this is true for the whole range of plastic properties of crystals and crystal aggregates. Included are all metals and alloys since these, as is well known from x-ray evidence, are crystalline even though their external shape rarely gives any
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