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 is seen from (1), § 6, to be an invariant. If we multiply this by the invariants

we obtain the invariants

and the constitutive relations are obtained by making the first of these equal to &mu; times the third, and the second equal to &epsilon; times the fourth, where &epsilon; and &mu; are invariants. These, however, are not the only constitutive relations which remain invariant, for we may obtain the two integral invariants

A set of constitutive relations given by two linear relations between