The linear @F02515@ relation: \[\log _{10}(\frac{k_{\text{s}}}{k_{0}}) = m\ Y\] expressing the dependence of the rate of @S05762@ of a substrate on @I03215@ of the solvent. The @O04322@ \(k_{0}\) applies to the reference solvent (ethanol–water, 80:20, v/v) and \(k_{\text{s}}\) to the solvent s, both at \(25\ ^{\,\unicode{x26ac}}\text{C}\). The parameter \(m\) is characteristic of the substrate and is assigned the value unity for tert-butyl chloride. The value \(Y\) is intended to be a quantitative measure of the @I03215@ of the solvents. The equation was later extended to the form: \[\log _{10}(\frac{k_{\text{s}}}{k_{0}}) = m\ Y+l\ N\] where \(N\) is the @N04251@ of the solvent and \(l\) its susceptibility parameter. The equation has also been applied to reactions other than @S05762@.