In A Treatise on Electricity and Magnetism, an 1873 treatise on electromagnetism written by James Clerk Maxwell, twelve general equations of the electromagnetic field are listed and these include the eight that are listed in the 1865 paper. His theoretical investigations of the electromagnetic field was guided by the notions of work, energy, potential, the principle of conservation of energy, and Lagrangian dynamics. All the principal equations concerning Maxwell's electromagnetic theory are recapitulated in Chapter IX of Part IV. At the end of this chapter, all the equations are listed and set in quaternion form. The first two equations [ A ] and [ B ] relates the electric scalar potential and magnetic vector potential to the electric and magnetic fields. The third equation [ C ] relates the electromagnetic field to electromagnetic force. The rest of the equations [ D ] to [ L ] relates the electromagnetic field to material data: the current and charge densities as well as the material medium.