The linear @[email protected] relation: \[\log _{10}(\frac{k_{\text{s}}}{k_{0}}) = m\ Y\] expressing the dependence of the rate of @[email protected] of a substrate on @[email protected] of the solvent. The @[email protected] \(k_{0}\) 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 @[email protected] 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 @[email protected] of the solvent and \(l\) its susceptibility parameter. The equation has also been applied to reactions other than @[email protected]