Introduction, definitions of algebras, illustrations -- Linear sets of elements of an algebra -- Invariant sub-algebras, direct sum, reducibility, difference algebras -- Nilpotent and semi-simple algebras; idempotent elements -- Division algebras -- Structure of algebras -- Characteristic matrices, determinants, and equations; minimum and rank equations -- The principal theorem on algebras -- Integral algebraic numbers -- The arithmetic of an algebra -- Fields -- Appendix I. Division algebras of order n² -- II. Determination of all division algebras of order 9; miscellaneous general theorems on division algebras -- III. Statement of further results and unsolved problems