In mathematics, the biquadratic power or root of a quantity is its fourth power or root (see ALGEBRA); a biquadratic equation is an equation in which the highest power of the unknown is the fourth (see EQUATION: Bgquadratic).

An explanation of the failure of the usual dilution law in these cases may be given if we remember that, while the electric forces between bodies like undissociated molecules, each associated with equal and opposite charges, will vary inversely as the fourth power of the distance, the forces between dissociated ions, each carrying one charge only, will be inversely proportional to the square of the distance.

Stefan's law of radiation according to the fourth power of the temperature is too difficult to pursue, but if we are content with cognate results we can follow them out mathematically in a hypothetical law of the first power.