ONE aspect of the solution problem which has lately come into prominence is the effect of the forces acting between electrically charged ions. Faraday was much impressed by what he called “the enormous electric power of each particle or atom of matter,” that is, the large size of the ionic charge. Helmholtz in his Faraday Lecture gave a calculation showing that the attractive force between the electrical charges associated with equivalent quantities of oxygen and hydrogen is 71,000 billion times greater than the gravitational attraction between these masses. It would seem obvious that such forces as these must affect the behaviour of ions. However, the difficulties in the way of a mathematical analysis of the effects of the electrical forces between ions in solution were considerable, and the problem was not attacked successfully until thirty years later.